diff --git a/gslice/gslice.go b/gslice/gslice.go
index 5a2a3a8..3eca1ca 100644
--- a/gslice/gslice.go
+++ b/gslice/gslice.go
@@ -875,8 +875,9 @@ func Union[S ~[]T, T comparable](ss ...S) S {
 	if len(ss) == 1 {
 		return Uniq(ss[0])
 	}
-	members := set.New[T]()
-	ret := S{} // TODO: Guess a cap.
+	size := Sum(Map(ss, func(s S) int { return len(s) }))
+	members := set.NewWithCap[T](size)
+	ret := make(S, 0, size/2)
 	for _, s := range ss {
 		for _, v := range s {
 			if members.Add(v) {
diff --git a/gslice/gslice_bench_test.go b/gslice/gslice_bench_test.go
index b6c3165..f7ab6e0 100644
--- a/gslice/gslice_bench_test.go
+++ b/gslice/gslice_bench_test.go
@@ -19,6 +19,7 @@ import (
 	"strconv"
 	"testing"
 
+	"github.com/bytedance/gg/collection/set"
 	"github.com/bytedance/gg/internal/iter"
 )
 
@@ -107,3 +108,106 @@ func BenchmarkShuffle_Parallel(b *testing.B) {
 		})
 	})
 }
+
+func oldUnion[S ~[]T, T comparable](ss ...S) S {
+	if len(ss) == 0 {
+		return S{}
+	}
+	if len(ss) == 1 {
+		return Uniq(ss[0])
+	}
+	members := set.New[T]()
+	ret := S{} // TODO: Guess a cap.
+	for _, s := range ss {
+		for _, v := range s {
+			if members.Add(v) {
+				ret = append(ret, v)
+			}
+		}
+	}
+	return ret
+}
+
+func BenchmarkUnion(b *testing.B) {
+	// 1. all different
+	ss1 := [][]int{
+		Range(0, 10),
+		Range(10, 20),
+	}
+	b.Run("new-union-diff-2-10", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			Union(ss1...)
+		}
+	})
+	b.Run("old-union-diff-2-10", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			oldUnion(ss1...)
+		}
+	})
+	ss2 := [][]int{
+		Range(0, 100),
+		Range(100, 200),
+		Range(200, 300),
+		Range(300, 400),
+		Range(400, 500),
+	}
+	b.Run("new-union-diff-5-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			Union(ss2...)
+		}
+	})
+	b.Run("old-union-diff-5-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			oldUnion(ss2...)
+		}
+	})
+
+	// 2. all same
+	ss3 := [][]int{
+		Repeat(0, 10),
+		Repeat(0, 10),
+	}
+	b.Run("new-union-same-2-10", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			Union(ss3...)
+		}
+	})
+	b.Run("old-union-same-2-10", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			oldUnion(ss3...)
+		}
+	})
+	ss4 := [][]int{
+		Repeat(0, 100),
+		Repeat(0, 100),
+		Repeat(0, 100),
+		Repeat(0, 100),
+		Repeat(0, 100),
+	}
+	b.Run("new-union-same-5-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			Union(ss4...)
+		}
+	})
+	b.Run("old-union-same-5-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			oldUnion(ss4...)
+		}
+	})
+
+	// 3. half different
+	ss5 := [][]int{
+		Range(0, 100),
+		Range(50, 150),
+	}
+	b.Run("new-union-half-2-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			Union(ss5...)
+		}
+	})
+	b.Run("old-union-half-2-100", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			oldUnion(ss5...)
+		}
+	})
+}