diff --git a/feature_reflect.go b/feature_reflect.go
index 4b0ab92..e1a7789 100644
--- a/feature_reflect.go
+++ b/feature_reflect.go
@@ -38,7 +38,7 @@ func writeToStream(val interface{}, stream *Stream, encoder Encoder) {
 
 type DecoderFunc func(ptr unsafe.Pointer, iter *Iterator)
 type EncoderFunc func(ptr unsafe.Pointer, stream *Stream)
-type ExtensionFunc func(typ reflect.Type, field *reflect.StructField) ([]string, DecoderFunc)
+type ExtensionFunc func(typ reflect.Type, field *reflect.StructField) ([]string, EncoderFunc, DecoderFunc)
 
 type funcDecoder struct {
 	fun DecoderFunc
diff --git a/feature_reflect_object.go b/feature_reflect_object.go
index 722567a..cd7da3c 100644
--- a/feature_reflect_object.go
+++ b/feature_reflect_object.go
@@ -13,12 +13,16 @@ func encoderOfStruct(typ reflect.Type) (Encoder, error) {
 	structEncoder_ := &structEncoder{}
 	for i := 0; i < typ.NumField(); i++ {
 		field := typ.Field(i)
+		fieldEncoderKey := fmt.Sprintf("%s/%s", typ.String(), field.Name)
 		var extensionProvidedFieldNames []string
 		for _, extension := range extensions {
-			alternativeFieldNames, _ := extension(typ, &field)
+			alternativeFieldNames, fun, _ := extension(typ, &field)
 			if alternativeFieldNames != nil {
 				extensionProvidedFieldNames = alternativeFieldNames
 			}
+			if fun != nil {
+				fieldEncoders[fieldEncoderKey] = &funcEncoder{fun}
+			}
 		}
 		tagParts := strings.Split(field.Tag.Get("json"), ",")
 		// if fieldNames set by extension, use theirs, otherwise try tags
@@ -29,9 +33,9 @@ func encoderOfStruct(typ reflect.Type) (Encoder, error) {
 				omitempty = true
 			}
 		}
-		var encoder Encoder
+		encoder := fieldEncoders[fieldEncoderKey]
 		var err error
-		if len(fieldNames) > 0 {
+		if encoder == nil && len(fieldNames) > 0 {
 			encoder, err = encoderOfType(field.Type)
 			if err != nil {
 				return prefix(fmt.Sprintf("{%s}", field.Name)).addToEncoder(encoder, err)
@@ -59,7 +63,7 @@ func decoderOfStruct(typ reflect.Type) (Decoder, error) {
 		fieldDecoderKey := fmt.Sprintf("%s/%s", typ.String(), field.Name)
 		var extensionProviedFieldNames []string
 		for _, extension := range extensions {
-			alternativeFieldNames, fun := extension(typ, &field)
+			alternativeFieldNames, _, fun := extension(typ, &field)
 			if alternativeFieldNames != nil {
 				extensionProviedFieldNames = alternativeFieldNames
 			}
@@ -112,8 +116,8 @@ func calcFieldNames(originalFieldName string, tagProvidedFieldName string, exten
 }
 
 func EnableUnexportedStructFieldsSupport() {
-	RegisterExtension(func(type_ reflect.Type, field *reflect.StructField) ([]string, DecoderFunc) {
-		return []string{field.Name}, nil
+	RegisterExtension(func(type_ reflect.Type, field *reflect.StructField) ([]string, EncoderFunc, DecoderFunc) {
+		return []string{field.Name}, nil, nil
 	})
 }
 
diff --git a/jsoniter_customize_test.go b/jsoniter_customize_test.go
index 9b502a9..8bb9984 100644
--- a/jsoniter_customize_test.go
+++ b/jsoniter_customize_test.go
@@ -86,22 +86,28 @@ type TestObject1 struct {
 }
 
 func Test_customize_field_by_extension(t *testing.T) {
-	RegisterExtension(func(type_ reflect.Type, field *reflect.StructField) ([]string, DecoderFunc) {
+	should := require.New(t)
+	RegisterExtension(func(type_ reflect.Type, field *reflect.StructField) ([]string, EncoderFunc, DecoderFunc) {
 		if type_.String() == "jsoniter.TestObject1" && field.Name == "field1" {
-			return []string{"field-1"}, func(ptr unsafe.Pointer, iter *Iterator) {
+			encode := func(ptr unsafe.Pointer, stream *Stream) {
+				str := *((*string)(ptr))
+				val, _ := strconv.Atoi(str)
+				stream.WriteInt(val)
+			}
+			decode := func(ptr unsafe.Pointer, iter *Iterator) {
 				*((*string)(ptr)) = strconv.Itoa(iter.ReadInt())
 			}
+			return []string{"field-1"}, encode, decode
 		}
-		return nil, nil
+		return nil, nil, nil
 	})
 	obj := TestObject1{}
-	err := Unmarshal([]byte(`{"field-1": 100}`), &obj)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if obj.field1 != "100" {
-		t.Fatal(obj.field1)
-	}
+	err := UnmarshalFromString(`{"field-1": 100}`, &obj)
+	should.Nil(err)
+	should.Equal("100", obj.field1)
+	str, err := MarshalToString(obj)
+	should.Nil(err)
+	should.Equal(`{"field-1":100}`, str)
 }
 
 func Test_unexported_fields(t *testing.T) {