diff --git a/index/field/string_field_iterator_test.go b/index/field/string_field_iterator_test.go
new file mode 100644
index 0000000..f073134
--- /dev/null
+++ b/index/field/string_field_iterator_test.go
@@ -0,0 +1,65 @@
+package field
+
+import (
+	"testing"
+
+	"github.com/xichen2020/eventdb/index"
+	"github.com/xichen2020/eventdb/values/iterator"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStringFieldIterator(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	docIt := index.NewMockDocIDSetIterator(ctrl)
+	gomock.InOrder(
+		docIt.EXPECT().Next().Return(true),
+		docIt.EXPECT().DocID().Return(int32(3)),
+		docIt.EXPECT().Next().Return(true),
+		docIt.EXPECT().DocID().Return(int32(4)),
+		docIt.EXPECT().Next().Return(true),
+		docIt.EXPECT().DocID().Return(int32(7)),
+		docIt.EXPECT().Next().Return(true),
+		docIt.EXPECT().DocID().Return(int32(10)),
+		docIt.EXPECT().Next().Return(true),
+		docIt.EXPECT().DocID().Return(int32(15)),
+		docIt.EXPECT().Next().Return(false).AnyTimes(),
+		docIt.EXPECT().Close(),
+	)
+
+	valsIt := iterator.NewMockForwardStringIterator(ctrl)
+	gomock.InOrder(
+		valsIt.EXPECT().Next().Return(true),
+		valsIt.EXPECT().Current().Return("a"),
+		valsIt.EXPECT().Next().Return(true),
+		valsIt.EXPECT().Current().Return("b"),
+		valsIt.EXPECT().Next().Return(true),
+		valsIt.EXPECT().Current().Return("c"),
+		valsIt.EXPECT().Next().Return(true),
+		valsIt.EXPECT().Current().Return("d"),
+		valsIt.EXPECT().Next().Return(true),
+		valsIt.EXPECT().Current().Return("e"),
+		valsIt.EXPECT().Err().Return(nil),
+		valsIt.EXPECT().Close(),
+	)
+
+	it := newStringFieldIterator(docIt, valsIt)
+	defer it.Close()
+
+	var (
+		expectedDocIDs = []int32{3, 4, 7, 10, 15}
+		expectedVals   = []string{"a", "b", "c", "d", "e"}
+		actualDocIDs   []int32
+		actualVals     []string
+	)
+	for it.Next() {
+		actualDocIDs = append(actualDocIDs, it.DocID())
+		actualVals = append(actualVals, it.Value())
+	}
+	require.NoError(t, it.Err())
+	require.Equal(t, expectedDocIDs, actualDocIDs)
+	require.Equal(t, expectedVals, actualVals)
+}