diff --git a/executor/insert_test.go b/executor/insert_test.go
index b2c29c195b298..1af7a0d89fee4 100644
--- a/executor/insert_test.go
+++ b/executor/insert_test.go
@@ -189,6 +189,13 @@ func (s *testSuite8) TestInsertOnDuplicateKey(c *C) {
 	tk.MustExec(`insert into t1 values(4,14),(5,15),(6,16),(7,17),(8,18) on duplicate key update b=b+10`)
 	c.Assert(tk.Se.AffectedRows(), Equals, uint64(7))
 	tk.CheckLastMessage("Records: 5  Duplicates: 2  Warnings: 0")
+
+	// reproduce insert on duplicate key update bug under new row format.
+	tk.MustExec(`drop table if exists t1`)
+	tk.MustExec(`create table t1(c1 decimal(6,4), primary key(c1))`)
+	tk.MustExec(`insert into t1 set c1 = 0.1`)
+	tk.MustExec(`insert into t1 set c1 = 0.1 on duplicate key update c1 = 1`)
+	tk.MustQuery(`select * from t1 use index(primary)`).Check(testkit.Rows(`1.0000`))
 }
 
 func (s *testSuite3) TestUpdateDuplicateKey(c *C) {
diff --git a/util/rowcodec/decoder.go b/util/rowcodec/decoder.go
index 08f8baab48277..5ef5ca3256a73 100644
--- a/util/rowcodec/decoder.go
+++ b/util/rowcodec/decoder.go
@@ -133,11 +133,13 @@ func (decoder *DatumMapDecoder) decodeColDatum(col *ColInfo, colData []byte) (ty
 		mysql.TypeBlob, mysql.TypeTinyBlob, mysql.TypeMediumBlob, mysql.TypeLongBlob:
 		d.SetBytes(colData)
 	case mysql.TypeNewDecimal:
-		_, dec, _, _, err := codec.DecodeDecimal(colData)
+		_, dec, precision, frac, err := codec.DecodeDecimal(colData)
 		if err != nil {
 			return d, err
 		}
 		d.SetMysqlDecimal(dec)
+		d.SetLength(precision)
+		d.SetFrac(frac)
 	case mysql.TypeDate, mysql.TypeDatetime, mysql.TypeTimestamp:
 		var t types.Time
 		t.SetType(uint8(col.Tp))
@@ -268,10 +270,18 @@ func (decoder *ChunkDecoder) decodeColToChunk(colIdx int, col *ColInfo, colData
 		mysql.TypeBlob, mysql.TypeTinyBlob, mysql.TypeMediumBlob, mysql.TypeLongBlob:
 		chk.AppendBytes(colIdx, colData)
 	case mysql.TypeNewDecimal:
-		_, dec, _, _, err := codec.DecodeDecimal(colData)
+		_, dec, _, frac, err := codec.DecodeDecimal(colData)
 		if err != nil {
 			return err
 		}
+		if col.Decimal != types.UnspecifiedLength && frac > col.Decimal {
+			to := new(types.MyDecimal)
+			err := dec.Round(to, col.Decimal, types.ModeHalfEven)
+			if err != nil {
+				return errors.Trace(err)
+			}
+			dec = to
+		}
 		chk.AppendMyDecimal(colIdx, dec)
 	case mysql.TypeDate, mysql.TypeDatetime, mysql.TypeTimestamp:
 		var t types.Time
diff --git a/util/rowcodec/rowcodec_test.go b/util/rowcodec/rowcodec_test.go
index 86f253769239b..336ff36264a27 100644
--- a/util/rowcodec/rowcodec_test.go
+++ b/util/rowcodec/rowcodec_test.go
@@ -363,8 +363,8 @@ func (s *testSuite) TestTypesNewRowCodec(c *C) {
 		{
 			8,
 			types.NewFieldType(mysql.TypeNewDecimal),
-			types.NewDecimalDatum(types.NewDecFromStringForTest("1.99")),
-			types.NewDecimalDatum(types.NewDecFromStringForTest("1.99")),
+			withFrac(4)(withLen(6)(types.NewDecimalDatum(types.NewDecFromStringForTest("11.9900")))),
+			withFrac(4)(withLen(6)(types.NewDecimalDatum(types.NewDecFromStringForTest("11.9900")))),
 			nil,
 			false,
 		},
@@ -775,4 +775,16 @@ var (
 	getDatumPoint = func(d types.Datum) *types.Datum {
 		return &d
 	}
+	withFrac = func(f int) func(d types.Datum) types.Datum {
+		return func(d types.Datum) types.Datum {
+			d.SetFrac(f)
+			return d
+		}
+	}
+	withLen = func(len int) func(d types.Datum) types.Datum {
+		return func(d types.Datum) types.Datum {
+			d.SetLength(len)
+			return d
+		}
+	}
 )