diff --git a/src/datastruct/sortedset/border.go b/src/datastruct/sortedset/border.go
new file mode 100644
index 0000000..e5d644e
--- /dev/null
+++ b/src/datastruct/sortedset/border.go
@@ -0,0 +1,89 @@
+package sortedset
+
+import (
+    "errors"
+    "strconv"
+)
+
+/*
+ * ScoreBorder is a struct represents `min` `max` parameter of redis command `ZRANGEBYSCORE`
+ * can accept:
+ *   int or float value, such as 2.718, 2, -2.718, -2 ...
+ *   exclusive int or float value, such as (2.718, (2, (-2.718, (-2 ...
+ *   infinity: +inf, -inf， inf(same as +inf)
+ */
+
+const (
+    negativeInf int8 = -1
+    positiveInf int8 = 1
+)
+
+type ScoreBorder struct {
+    Inf int8
+    Value float64
+    Exclude bool
+}
+
+func (border *ScoreBorder)greater(value float64)bool {
+    if border.Inf == negativeInf {
+        return false
+    } else if border.Inf == positiveInf {
+        return true
+    }
+    if border.Exclude {
+        return border.Value > value
+    } else {
+        return border.Value >= value
+    }
+}
+
+func (border *ScoreBorder)less(value float64)bool {
+    if border.Inf == negativeInf {
+        return true
+    } else if border.Inf == positiveInf {
+        return false
+    }
+    if border.Exclude {
+        return border.Value < value
+    } else {
+        return border.Value <= value
+    }
+}
+
+var positiveInfBorder = &ScoreBorder {
+    Inf: positiveInf,
+}
+
+var negativeInfBorder = &ScoreBorder {
+    Inf: negativeInf,
+}
+
+func ParseScoreBorder(s string)(*ScoreBorder, error) {
+    if s == "inf" || s == "+inf" {
+        return positiveInfBorder, nil
+    }
+    if s == "-inf" {
+        return negativeInfBorder, nil
+    }
+    if s[0] == '(' {
+        value, err := strconv.ParseFloat(s[1:], 64)
+        if err != nil {
+            return nil, errors.New("ERR min or max is not a float")
+        }
+        return &ScoreBorder{
+            Inf: 0,
+            Value: value,
+            Exclude: true,
+        }, nil
+    } else {
+        value, err := strconv.ParseFloat(s, 64)
+        if err != nil {
+            return nil, errors.New("ERR min or max is not a float")
+        }
+        return &ScoreBorder{
+            Inf: 0,
+            Value: value,
+            Exclude: false,
+        }, nil
+    }
+}
\ No newline at end of file
diff --git a/src/datastruct/sortedset/skiplist.go b/src/datastruct/sortedset/skiplist.go
new file mode 100644
index 0000000..2820a51
--- /dev/null
+++ b/src/datastruct/sortedset/skiplist.go
@@ -0,0 +1,290 @@
+package sortedset
+
+import "math/rand"
+
+const (
+    maxLevel = 16
+)
+
+
+type Element struct {
+    Member string
+    Score  float64
+}
+
+// level aspect of a Node
+type Level struct {
+    forward *Node // forward node has greater score
+    span int64
+}
+
+type Node struct {
+    Element
+    backward *Node
+    level []*Level // level[0] is base level
+}
+
+type skiplist struct {
+    header *Node
+    tail *Node
+    length int64
+    level int16
+}
+
+func makeNode(level int16, score float64, member string)*Node {
+    n := &Node{
+        Element: Element{
+            Score:  score,
+            Member: member,
+        },
+        level: make([]*Level, level),
+    }
+    for i := range n.level {
+        n.level[i] = new(Level)
+    }
+    return n
+}
+
+func makeSkiplist()*skiplist {
+    return &skiplist{
+        level:  1,
+        header: makeNode(maxLevel, 0, ""),
+    }
+}
+
+func randomLevel() int16 {
+    level := int16(1)
+    for float32(rand.Int31()&0xFFFF) < (0.25 * 0xFFFF) {
+        level++
+    }
+    if level < maxLevel {
+        return level
+    }
+    return maxLevel
+}
+
+func (skiplist *skiplist)insert(member string, score float64)*Node {
+    update := make([]*Node, maxLevel) // link new node with node in `update`
+    rank := make([]int64, maxLevel)
+
+    // find position to insert
+    node := skiplist.header
+    for i := skiplist.level - 1; i >= 0; i-- {
+        if i == skiplist.level - 1 {
+            rank[i] = 0
+        } else {
+            rank[i] = rank[i + 1] // store rank that is crossed to reach the insert position
+        }
+        if node.level[i] != nil {
+            // traverse the skip list
+            for node.level[i].forward != nil &&
+                (node.level[i].forward.Score < score ||
+                    (node.level[i].forward.Score == score && node.level[i].forward.Member < member)) { // same score, different key
+                rank[i] += node.level[i].span
+                node = node.level[i].forward
+            }
+        }
+        update[i] = node
+    }
+
+    level := randomLevel()
+    // extend skiplist level
+    if level > skiplist.level {
+        for i := skiplist.level; i < level; i++ {
+            rank[i] = 0
+            update[i] = skiplist.header
+            update[i].level[i].span = skiplist.length
+        }
+        skiplist.level = level
+    }
+
+    // make node and link into skiplist
+    node = makeNode(level, score, member)
+    for i := int16(0); i < level; i++ {
+        node.level[i].forward = update[i].level[i].forward
+        update[i].level[i].forward = node
+
+        // update span covered by update[i] as node is inserted here
+        node.level[i].span = update[i].level[i].span - (rank[0] - rank[i])
+        update[i].level[i].span = (rank[0] - rank[i]) + 1
+    }
+
+    // increment span for untouched levels
+    for i := level; i < skiplist.level; i++ {
+        update[i].level[i].span++
+    }
+
+    // set backward node
+    if update[0] == skiplist.header {
+        node.backward = nil
+    } else {
+        node.backward = update[0]
+    }
+    if node.level[0].forward != nil {
+        node.level[0].forward.backward = node
+    } else {
+        skiplist.tail = node
+    }
+    skiplist.length++
+    return node
+}
+
+/*
+ * param node: node to delete
+ * param update: backward node (of target) or last node of each level
+ */
+func (skiplist *skiplist) removeNode(node *Node, update []*Node) {
+    for i := int16(0); i < skiplist.level; i++ {
+        if update[i].level[i].forward == node {
+            update[i].level[i].span += node.level[i].span - 1
+            update[i].level[i].forward = node.level[i].forward
+        } else {
+            update[i].level[i].span--
+        }
+    }
+    if node.level[0].forward != nil {
+        node.level[0].forward.backward = node.backward
+    } else {
+        skiplist.tail = node.backward
+    }
+    for skiplist.level > 1 && skiplist.header.level[skiplist.level-1].forward == nil {
+        skiplist.level--
+    }
+    skiplist.length--
+}
+
+/*
+ * return: has found and removed node
+ */
+func (skiplist *skiplist) remove(member string, score float64)bool  {
+    /*
+     * find backward node (of target) or last node of each level
+     * their forward need to be updated
+     */
+    update := make([]*Node, maxLevel)
+    node := skiplist.header
+    for i := skiplist.level - 1; i >= 0; i-- {
+        for node.level[i].forward != nil &&
+            (node.level[i].forward.Score < score ||
+                (node.level[i].forward.Score == score &&
+                    node.level[i].forward.Member < member)) {
+            node = node.level[i].forward
+        }
+        update[i] = node
+    }
+    node = node.level[0].forward
+    if node != nil && score == node.Score && node.Member == member {
+        skiplist.removeNode(node, update)
+        // free x
+        return true
+    }
+    return false
+}
+
+/*
+ * return: 1 based rank, 0 means member not found
+ */
+func (skiplist *skiplist) getRank(member string, score float64)int64 {
+    var rank int64 = 0
+    x := skiplist.header
+    for i := skiplist.level - 1; i >= 0; i-- {
+        for x.level[i].forward != nil &&
+            (x.level[i].forward.Score < score ||
+                (x.level[i].forward.Score == score &&
+                    x.level[i].forward.Member <= member)) {
+            rank += x.level[i].span
+            x = x.level[i].forward
+        }
+
+        /* x might be equal to zsl->header, so test if obj is non-NULL */
+        if x.Member == member {
+            return rank
+        }
+    }
+    return 0
+}
+
+/*
+ * 1-based rank
+ */
+func (skiplist *skiplist) getByRank(rank int64)*Node {
+    var i int64 = 0
+    n := skiplist.header
+    // scan from top level
+    for level := skiplist.level - 1; level >= 0; level-- {
+        for n.level[level].forward != nil && (i+n.level[level].span) <= rank {
+            i += n.level[level].span
+            n = n.level[level].forward
+        }
+        if i == rank {
+            return n
+        }
+    }
+    return nil
+}
+
+/*
+ * return removed elements
+ */
+func (skiplist *skiplist) RemoveRangeByScore(min *ScoreBorder, max *ScoreBorder)(removed []*Element) {
+    update := make([]*Node, maxLevel)
+    removed = make([]*Element, 0)
+    // find backward nodes (of target range) or last node of each level
+    node := skiplist.header
+    for i := skiplist.level - 1; i >= 0; i-- {
+        for node.level[i].forward != nil {
+            if min.less(node.level[i].forward.Score) { // already in range
+                break
+            }
+            node = node.level[i].forward
+        }
+        update[i] = node
+    }
+
+    // node is the first one within range
+    node = node.level[0].forward
+
+    // remove nodes in range
+    for node != nil {
+        if !max.greater(node.Score) { // already out of range
+            break
+        }
+        next := node.level[0].forward
+        removedElement := node.Element
+        removed = append(removed, &removedElement)
+        skiplist.removeNode(node, update)
+        node = next
+    }
+    return removed
+}
+
+// 1-based rank, including start, exclude stop
+func (skiplist *skiplist) RemoveRangeByRank(start int64, stop int64)(removed []*Element) {
+    var i int64 = 0  // rank of iterator
+    update := make([]*Node, maxLevel)
+    removed = make([]*Element, 0)
+
+    // scan from top level
+    node := skiplist.header
+    for level := skiplist.level - 1; level >= 0; level-- {
+        for node.level[level].forward != nil && (i+node.level[level].span) < start {
+            i += node.level[level].span
+            node = node.level[level].forward
+        }
+        update[level] = node
+    }
+
+    i++
+    node = node.level[0].forward // first node in range
+
+    // remove nodes in range
+    for node != nil && i < stop {
+        next := node.level[0].forward
+        removedElement := node.Element
+        removed = append(removed, &removedElement)
+        skiplist.removeNode(node, update)
+        node = next
+        i++
+    }
+    return removed
+}
diff --git a/src/datastruct/sortedset/sortedset.go b/src/datastruct/sortedset/sortedset.go
new file mode 100644
index 0000000..48ba062
--- /dev/null
+++ b/src/datastruct/sortedset/sortedset.go
@@ -0,0 +1,203 @@
+package sortedset
+
+import (
+    "strconv"
+)
+
+type SortedSet struct {
+    dict map[string]*Element
+    skiplist *skiplist
+}
+
+func Make()*SortedSet {
+    return &SortedSet{
+        dict: make(map[string]*Element),
+        skiplist: makeSkiplist(),
+    }
+}
+
+/*
+ * return: has inserted new node
+ */
+func (sortedSet *SortedSet)Add(member string, score float64)bool {
+    element, ok := sortedSet.dict[member]
+    sortedSet.dict[member] = &Element{
+        Member: member,
+        Score:  score,
+    }
+    if ok {
+        if score != element.Score {
+            sortedSet.skiplist.remove(member, score)
+            sortedSet.skiplist.insert(member, score)
+        }
+        return false
+    } else {
+        sortedSet.skiplist.insert(member, score)
+        return true
+    }
+}
+
+func (sortedSet *SortedSet) Len()int64 {
+    return int64(len(sortedSet.dict))
+}
+
+func (sortedSet *SortedSet) Get(member string) (element *Element, ok bool) {
+    element, ok = sortedSet.dict[member]
+    if !ok {
+        return nil, false
+    }
+    return element, true
+}
+
+func (sortedSet *SortedSet) Remove(member string)bool {
+    v, ok := sortedSet.dict[member]
+    if ok {
+        sortedSet.skiplist.remove(member, v.Score)
+        delete(sortedSet.dict, member)
+        return true
+    }
+    return false
+}
+
+/**
+ * get 0-based rank
+ */
+func (sortedSet *SortedSet) GetRank(member string, desc bool) (rank int64) {
+    element, ok := sortedSet.dict[member]
+    if !ok {
+        return -1
+    }
+    r := sortedSet.skiplist.getRank(member, element.Score)
+    if desc {
+        r = sortedSet.skiplist.length - r
+    } else {
+        r--
+    }
+    return r
+}
+
+/**
+ * traverse [start, stop), 0-based rank
+ */
+func (sortedSet *SortedSet) ForEach(start int64, stop int64, desc bool, consumer func(element *Element)bool) {
+    size := int64(sortedSet.Len())
+    if start < 0 || start >= size {
+        panic("illegal start " + strconv.FormatInt(start, 10))
+    }
+    if stop < start || stop > size {
+        panic("illegal end " + strconv.FormatInt(stop, 10))
+    }
+
+    // find start node
+    var node *Node
+    if desc {
+        node = sortedSet.skiplist.tail
+        if start > 0 {
+            node = sortedSet.skiplist.getByRank(int64(size - start))
+        }
+    } else {
+        node = sortedSet.skiplist.header.level[0].forward
+        if start > 0 {
+            node = sortedSet.skiplist.getByRank(int64(start + 1))
+        }
+    }
+
+    sliceSize := int(stop - start)
+    for i := 0; i < sliceSize; i++ {
+        if !consumer(&node.Element) {
+            break
+        }
+        if desc {
+            node = node.backward
+        } else {
+            node = node.level[0].forward
+        }
+    }
+}
+
+/**
+ * return [start, stop), 0-based rank
+ * assert start in [0, size), stop in [start, size]
+ */
+func (sortedSet *SortedSet) Range(start int64, stop int64, desc bool)[]*Element {
+    sliceSize := int(stop - start)
+    slice := make([]*Element, sliceSize)
+    i := 0
+    sortedSet.ForEach(start, stop, desc, func(element *Element)bool {
+        slice[i] = element
+        i++
+        return true
+    })
+    return slice
+}
+
+func (sortedSet *SortedSet) Count(min *ScoreBorder, max *ScoreBorder)int64 {
+    var i int64 = 0
+    // ascending order
+    sortedSet.ForEach(0, sortedSet.Len(), false, func(element *Element) bool {
+        gtMin := min.less(element.Score) // greater than min
+        if !gtMin {
+            // has not into range, continue foreach
+            return true
+        }
+        ltMax := max.greater(element.Score) // less than max
+        if !ltMax {
+            // break through score border, break foreach
+            return false
+        }
+        // gtMin && ltMax
+        i++
+        return true
+    })
+    return i
+}
+
+/*
+ * param limit: <0 means no limit
+ */
+func (sortedSet *SortedSet) RangeByScore(min *ScoreBorder, max *ScoreBorder, offset int64, limit int64, desc bool)[]*Element {
+    if limit == 0 || offset < 0{
+        return make([]*Element, 0)
+    }
+    slice := make([]*Element, 0)
+    var skipped int64 = 0
+    sortedSet.ForEach(0, sortedSet.Len(), desc, func(element *Element)bool {
+        gtMin := min.less(element.Score) // greater than min
+        ltMax := max.greater(element.Score) // less than max
+        if gtMin && ltMax { // in score range
+            if skipped < offset {
+                skipped++
+                return true
+            }
+            slice = append(slice, element)
+            if len(slice) == int(limit) { // reach limit
+                return false
+            }
+        }
+        if (desc && !gtMin) || (!desc && !ltMax) {
+            return false // break through score border
+        }
+        return true
+    })
+    return slice
+}
+
+func (sortedSet *SortedSet) RemoveByScore(min *ScoreBorder, max *ScoreBorder)int64 {
+    removed := sortedSet.skiplist.RemoveRangeByScore(min, max)
+    for _, element := range removed {
+        delete(sortedSet.dict, element.Member)
+    }
+    return int64(len(removed))
+}
+
+
+/*
+ * 0-based rank, [start, stop)
+ */
+func (sortedSet *SortedSet) RemoveByRank(start int64, stop int64)int64 {
+    removed := sortedSet.skiplist.RemoveRangeByRank(start + 1, stop + 1)
+    for _, element := range removed {
+        delete(sortedSet.dict, element.Member)
+    }
+    return int64(len(removed))
+}
\ No newline at end of file
diff --git a/src/db/db.go b/src/db/db.go
index 8384983..aa97269 100644
--- a/src/db/db.go
+++ b/src/db/db.go
@@ -117,6 +117,21 @@ func MakeCmdMap()map[string]CmdFunc {
     cmdMap["sdiffstore"] = SDiffStore
     cmdMap["srandmember"] = SRandMember
 
+    cmdMap["zadd"] = ZAdd
+    cmdMap["zscore"] = ZScore
+    cmdMap["zincrby"] = ZIncrBy
+    cmdMap["zrank"] = ZRank
+    cmdMap["zcount"] = ZCount
+    cmdMap["zrevrank"] = ZRevRank
+    cmdMap["zcard"] = ZCard
+    cmdMap["zrange"] = ZRange
+    cmdMap["zrevrange"] = ZRevRange
+    cmdMap["zrangebyscore"] = ZRangeByScore
+    cmdMap["zrevrangebyscore"] = ZRevRangeByScore
+    cmdMap["zrem"] = ZRem
+    cmdMap["zremrangebyscore"] = ZRemRangeByScore
+    cmdMap["zremrangebyrank"] = ZRemRangeByRank
+
     return cmdMap
 }
 
diff --git a/src/db/sortedset.go b/src/db/sortedset.go
new file mode 100644
index 0000000..0d4cf21
--- /dev/null
+++ b/src/db/sortedset.go
@@ -0,0 +1,602 @@
+package db
+
+import (
+    SortedSet "github.com/HDT3213/godis/src/datastruct/sortedset"
+    "github.com/HDT3213/godis/src/interface/redis"
+    "github.com/HDT3213/godis/src/redis/reply"
+    "strconv"
+    "strings"
+)
+
+func ZAdd(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 3 || len(args) % 2 != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zadd' command")
+    }
+    key := string(args[0])
+    size := (len(args) - 1) / 2
+    elements := make([]*SortedSet.Element, size)
+    for i := 0; i < size; i++ {
+        scoreValue := args[2 * i + 1]
+        member := string(args[2 * i + 2])
+        score, err := strconv.ParseFloat(string(scoreValue), 64)
+        if err != nil {
+            return reply.MakeErrReply("ERR value is not a valid float")
+        }
+        elements[i] = &SortedSet.Element{
+            Member:member,
+            Score:score,
+        }
+    }
+
+    // lock
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    entity, exists := db.Get(key)
+    if !exists {
+        entity = &DataEntity{
+            Code: SortedSetCode,
+            Data: SortedSet.Make(),
+        }
+        db.Data.Put(key, entity)
+    }
+
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // insert
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    i := 0
+    for _, e := range elements {
+        if sortedSet.Add(e.Member, e.Score) {
+            i++
+        }
+    }
+
+    return reply.MakeIntReply(int64(i))
+}
+
+func ZScore(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zscore' command")
+    }
+    key := string(args[0])
+    member := string(args[1])
+
+    // get entity
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.NullBulkReply{}
+    }
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    element, exists := sortedSet.Get(member)
+    if !exists {
+        return &reply.NullBulkReply{}
+    }
+    value := strconv.FormatFloat(element.Score, 'f', -1, 64)
+    return reply.MakeBulkReply([]byte(value))
+}
+
+func ZRank(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrank' command")
+    }
+    key := string(args[0])
+    member := string(args[1])
+
+    // get entity
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.NullBulkReply{}
+    }
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    rank := sortedSet.GetRank(member, false)
+    if rank < 0 {
+        return &reply.NullBulkReply{}
+    }
+    return reply.MakeIntReply(rank)
+}
+
+func ZRevRank(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrevrank' command")
+    }
+    key := string(args[0])
+    member := string(args[1])
+
+    // get entity
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.NullBulkReply{}
+    }
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    rank := sortedSet.GetRank(member, true)
+    if rank < 0 {
+        return &reply.NullBulkReply{}
+    }
+    return reply.MakeIntReply(rank)
+}
+
+func ZCard(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 1 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zcard' command")
+    }
+    key := string(args[0])
+
+    // get entity
+    entity, exists := db.Get(key)
+    if !exists {
+        return reply.MakeIntReply(0)
+    }
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    return reply.MakeIntReply(int64(sortedSet.Len()))
+}
+
+func ZRange(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 3 && len(args) != 4 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrange' command")
+    }
+    withScores := false
+    if len(args) == 4 {
+        if strings.ToUpper(string(args[3])) != "WITHSCORES" {
+            return reply.MakeErrReply("syntax error")
+        } else {
+            withScores = true
+        }
+    }
+    key := string(args[0])
+    start, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    stop, err := strconv.ParseInt(string(args[2]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    return range0(db, key, start, stop, withScores, false)
+}
+
+func ZRevRange(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) != 3 && len(args) != 4 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrevrange' command")
+    }
+    withScores := false
+    if len(args) == 4 {
+        if string(args[3]) != "WITHSCORES" {
+            return reply.MakeErrReply("syntax error")
+        } else {
+            withScores = true
+        }
+    }
+    key := string(args[0])
+    start, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    stop, err := strconv.ParseInt(string(args[2]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    return range0(db, key, start, stop, withScores, true)
+}
+
+func range0(db *DB, key string, start int64, stop int64, withScores bool, desc bool)redis.Reply {
+    // lock key
+    db.Locks.RLock(key)
+    defer db.Locks.RUnLock(key)
+
+    // get data
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.EmptyMultiBulkReply{}
+    }
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // compute index
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    size := sortedSet.Len() // assert: size > 0
+    if start < -1 * size {
+        start = 0
+    } else if start < 0 {
+        start = size + start
+    } else if start >= size {
+        return &reply.EmptyMultiBulkReply{}
+    }
+    if stop < -1 * size {
+        stop = 0
+    } else if stop < 0 {
+        stop = size + stop + 1
+    } else if stop < size {
+        stop = stop + 1
+    } else {
+        stop = size
+    }
+    if stop < start {
+        stop = start
+    }
+
+    // assert: start in [0, size - 1], stop in [start, size]
+    slice := sortedSet.Range(start, stop, desc)
+    if withScores {
+        result := make([][]byte, len(slice) * 2)
+        i := 0
+        for _, element := range slice {
+            result[i] = []byte(element.Member)
+            i++
+            scoreStr := strconv.FormatFloat(element.Score, 'f', -1, 64)
+            result[i] = []byte(scoreStr)
+            i++
+        }
+        return reply.MakeMultiBulkReply(result)
+    } else {
+        result := make([][]byte, len(slice))
+        i := 0
+        for _, element := range slice {
+            result[i] = []byte(element.Member)
+            i++
+        }
+        return reply.MakeMultiBulkReply(result)
+    }
+}
+
+func ZCount(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 3  {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zcount' command")
+    }
+    key := string(args[0])
+
+    min, err := SortedSet.ParseScoreBorder(string(args[1]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    max, err := SortedSet.ParseScoreBorder(string(args[2]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    db.Locks.RLock(key)
+    defer db.Locks.RUnLock(key)
+
+    // get data
+    entity, exists := db.Get(key)
+    if !exists {
+        return reply.MakeIntReply(0)
+    }
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    return reply.MakeIntReply(sortedSet.Count(min, max))
+}
+
+/*
+ * param limit: limit < 0 means no limit
+ */
+func rangeByScore0(db *DB, key string, min *SortedSet.ScoreBorder, max *SortedSet.ScoreBorder, offset int64, limit int64, withScores bool, desc bool)redis.Reply {
+    // lock key
+    db.Locks.RLock(key)
+    defer db.Locks.RUnLock(key)
+
+    // get data
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.EmptyMultiBulkReply{}
+    }
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    slice := sortedSet.RangeByScore(min, max, offset, limit, desc)
+    if withScores {
+        result := make([][]byte, len(slice) * 2)
+        i := 0
+        for _, element := range slice {
+            result[i] = []byte(element.Member)
+            i++
+            scoreStr := strconv.FormatFloat(element.Score, 'f', -1, 64)
+            result[i] = []byte(scoreStr)
+            i++
+        }
+        return reply.MakeMultiBulkReply(result)
+    } else {
+        result := make([][]byte, len(slice))
+        i := 0
+        for _, element := range slice {
+            result[i] = []byte(element.Member)
+            i++
+        }
+        return reply.MakeMultiBulkReply(result)
+    }
+}
+
+func ZRangeByScore(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 3  {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrangebyscore' command")
+    }
+    key := string(args[0])
+
+    min, err := SortedSet.ParseScoreBorder(string(args[1]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    max, err := SortedSet.ParseScoreBorder(string(args[2]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    withScores := false
+    var offset int64 = 0
+    var limit int64 = -1
+    if len(args) > 3 {
+        for i := 3; i < len(args); {
+            s := string(args[i])
+            if strings.ToUpper(s) == "WITHSCORES" {
+                withScores = true
+                i++
+            } else if strings.ToUpper(s) == "LIMIT" {
+                if len(args) < i+3 {
+                    return reply.MakeErrReply("ERR syntax error")
+                }
+                offset, err = strconv.ParseInt(string(args[i+1]), 10, 64)
+                if err != nil {
+                    return reply.MakeErrReply("ERR value is not an integer or out of range")
+                }
+                limit, err = strconv.ParseInt(string(args[i+2]), 10, 64)
+                if err != nil {
+                    return reply.MakeErrReply("ERR value is not an integer or out of range")
+                }
+                i += 3
+            } else {
+                return reply.MakeErrReply("ERR syntax error")
+            }
+        }
+    }
+    return rangeByScore0(db, key, min, max, offset, limit, withScores, false)
+}
+
+func ZRevRangeByScore(db *DB, args [][]byte)redis.Reply {
+    if len(args) < 3  {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrangebyscore' command")
+    }
+    key := string(args[0])
+
+    min, err := SortedSet.ParseScoreBorder(string(args[1]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    max, err := SortedSet.ParseScoreBorder(string(args[2]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    withScores := false
+    var offset int64 = 0
+    var limit int64 = -1
+    if len(args) > 3 {
+        for i := 3; i < len(args); {
+            s := string(args[i])
+            if strings.ToUpper(s) == "WITHSCORES" {
+                withScores = true
+                i++
+            } else if strings.ToUpper(s) == "LIMIT" {
+                if len(args) < i+3 {
+                    return reply.MakeErrReply("ERR syntax error")
+                }
+                offset, err = strconv.ParseInt(string(args[i+1]), 10, 64)
+                if err != nil {
+                    return reply.MakeErrReply("ERR value is not an integer or out of range")
+                }
+                limit, err = strconv.ParseInt(string(args[i+2]), 10, 64)
+                if err != nil {
+                    return reply.MakeErrReply("ERR value is not an integer or out of range")
+                }
+                i += 3
+            } else {
+                return reply.MakeErrReply("ERR syntax error")
+            }
+        }
+    }
+    return rangeByScore0(db, key, min, max, offset, limit, withScores, true)
+}
+
+func ZRemRangeByScore(db *DB, args [][]byte)redis.Reply  {
+    if len(args) != 3  {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zremrangebyscore' command")
+    }
+    key := string(args[0])
+
+    min, err := SortedSet.ParseScoreBorder(string(args[1]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    max, err := SortedSet.ParseScoreBorder(string(args[2]))
+    if err != nil {
+        return reply.MakeErrReply(err.Error())
+    }
+
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data
+    entity, exists := db.Get(key)
+    if !exists {
+        return &reply.EmptyMultiBulkReply{}
+    }
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    removed := sortedSet.RemoveByScore(min, max)
+    return reply.MakeIntReply(removed)
+}
+
+func ZRemRangeByRank(db *DB, args [][]byte)redis.Reply  {
+    if len(args) != 3  {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zremrangebyrank' command")
+    }
+    key := string(args[0])
+    start, err := strconv.ParseInt(string(args[1]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+    stop, err := strconv.ParseInt(string(args[2]), 10, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not an integer or out of range")
+    }
+
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get data
+    entity, exists := db.Get(key)
+    if !exists {
+        return reply.MakeIntReply(0)
+    }
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // compute index
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    size := sortedSet.Len() // assert: size > 0
+    if start < -1 * size {
+        start = 0
+    } else if start < 0 {
+        start = size + start
+    } else if start >= size {
+        return reply.MakeIntReply(0)
+    }
+    if stop < -1 * size {
+        stop = 0
+    } else if stop < 0 {
+        stop = size + stop + 1
+    } else if stop < size {
+        stop = stop + 1
+    } else {
+        stop = size
+    }
+    if stop < start {
+        stop = start
+    }
+
+    // assert: start in [0, size - 1], stop in [start, size]
+    removed := sortedSet.RemoveByRank(start, stop)
+    return reply.MakeIntReply(removed)
+}
+
+func ZRem(db *DB, args [][]byte)redis.Reply {
+    // parse args
+    if len(args) < 2 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zrem' command")
+    }
+    key := string(args[0])
+    fields := make([]string, len(args)-1)
+    fieldArgs := args[1:]
+    for i, v := range fieldArgs {
+        fields[i] = string(v)
+    }
+
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get entity
+    entity, exists := db.Get(key)
+    if !exists {
+        return reply.MakeIntReply(0)
+    }
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    var deleted int64 = 0
+    for _, field := range fields {
+        if sortedSet.Remove(field) {
+            deleted++
+        }
+    }
+    return reply.MakeIntReply(deleted)
+}
+
+func ZIncrBy(db *DB, args [][]byte)redis.Reply {
+    if len(args) != 3 {
+        return reply.MakeErrReply("ERR wrong number of arguments for 'zincrby' command")
+    }
+    key := string(args[0])
+    rawDelta := string(args[1])
+    field := string(args[2])
+    delta, err := strconv.ParseFloat(rawDelta, 64)
+    if err != nil {
+        return reply.MakeErrReply("ERR value is not a valid float")
+    }
+
+    db.Locks.Lock(key)
+    defer db.Locks.UnLock(key)
+
+    // get or init entity
+    entity, exists := db.Get(key)
+    if !exists {
+        entity = &DataEntity{
+            Code: SortedSetCode,
+            Data: SortedSet.Make(),
+        }
+        db.Data.Put(key, entity)
+    }
+
+    // check type
+    if entity.Code != SortedSetCode {
+        return &reply.WrongTypeErrReply{}
+    }
+
+    // put data
+    sortedSet, _ := entity.Data.(*SortedSet.SortedSet)
+    element, exists := sortedSet.Get(field)
+    if !exists {
+        sortedSet.Add(field, delta)
+        return reply.MakeBulkReply(args[1])
+    } else {
+        score := element.Score +  delta
+        sortedSet.Add(field, score)
+        bytes := []byte(strconv.FormatFloat(score, 'f', -1, 64))
+        return reply.MakeBulkReply(bytes)
+    }
+}
\ No newline at end of file