diff --git a/expression/expressions/cast.go b/expression/expressions/cast.go
index f2ecd1e8d0b54..8c29b6df39ed1 100644
--- a/expression/expressions/cast.go
+++ b/expression/expressions/cast.go
@@ -19,7 +19,6 @@ import (
 	"github.com/pingcap/tidb/context"
 	"github.com/pingcap/tidb/expression"
 	mysql "github.com/pingcap/tidb/mysqldef"
-	"github.com/pingcap/tidb/util/charset"
 	"github.com/pingcap/tidb/util/types"
 )
 
@@ -76,30 +75,9 @@ func (f *FunctionCast) Eval(ctx context.Context, args map[interface{}]interface{
 	if value == nil {
 		return nil, nil
 	}
-
-	// TODO: we need a better function convert between any two types according to FieldType.
-	// Not only check Type, but also consider Flen/Decimal/Charset and so on.
 	nv, err := types.Convert(value, f.Tp)
 	if err != nil {
 		return nil, err
 	}
-	if f.Tp.Tp == mysql.TypeString && f.Tp.Charset == charset.CharsetBin {
-		nv = []byte(nv.(string))
-	}
-	if f.Tp.Flen != types.UnspecifiedLength {
-		switch f.Tp.Tp {
-		case mysql.TypeString:
-			v := nv.(string)
-			if len(v) > int(f.Tp.Flen) {
-				v = v[:f.Tp.Flen]
-			}
-			return v, nil
-		}
-	}
-	if f.Tp.Tp == mysql.TypeLonglong {
-		if mysql.HasUnsignedFlag(f.Tp.Flag) {
-			return uint64(nv.(int64)), nil
-		}
-	}
 	return nv, nil
 }
diff --git a/util/types/convert.go b/util/types/convert.go
index d725015e1774b..0cf14d05c63d9 100644
--- a/util/types/convert.go
+++ b/util/types/convert.go
@@ -19,6 +19,7 @@ import (
 
 	"github.com/juju/errors"
 	mysql "github.com/pingcap/tidb/mysqldef"
+	"github.com/pingcap/tidb/util/charset"
 )
 
 // InvConv returns a failed convertion error.
@@ -71,6 +72,10 @@ func Convert(val interface{}, target *FieldType) (v interface{}, err error) { //
 		}
 		// TODO: consider target.Charset/Collate
 		x = truncateStr(x, target.Flen)
+		if target.Charset == charset.CharsetBin {
+			bx := []byte(x)
+			return bx, nil
+		}
 		return x, nil
 	case mysql.TypeBlob:
 		x, err := ToString(val)
@@ -78,6 +83,10 @@ func Convert(val interface{}, target *FieldType) (v interface{}, err error) { //
 			return InvConv(val, tp)
 		}
 		x = truncateStr(x, target.Flen)
+		if target.Charset == charset.CharsetBin {
+			bx := []byte(x)
+			return bx, nil
+		}
 		return x, nil
 	case mysql.TypeDuration:
 		fsp := mysql.DefaultFsp
@@ -127,6 +136,10 @@ func Convert(val interface{}, target *FieldType) (v interface{}, err error) { //
 		if err != nil {
 			return InvConv(val, tp)
 		}
+		// TODO: We should first convert to uint64 then check unsigned flag.
+		if mysql.HasUnsignedFlag(target.Flag) {
+			return uint64(x), nil
+		}
 		return x, nil
 	case mysql.TypeNewDecimal:
 		x, err := ToDecimal(val)
diff --git a/util/types/convert_test.go b/util/types/convert_test.go
index d5c69830b28e9..2c7aed98a92d4 100644
--- a/util/types/convert_test.go
+++ b/util/types/convert_test.go
@@ -36,6 +36,12 @@ func (s *testTypeConvertSuite) TestConvertType(c *C) {
 	v, err := Convert("123456", ft)
 	c.Assert(err, IsNil)
 	c.Assert(v, Equals, "1234")
+	ft = NewFieldType(mysql.TypeString)
+	ft.Flen = 4
+	ft.Charset = charset.CharsetBin
+	v, err = Convert("12345", ft)
+	c.Assert(err, IsNil)
+	c.Assert(v, DeepEquals, []byte("1234"))
 
 	ft = NewFieldType(mysql.TypeFloat)
 	ft.Flen = 5
@@ -111,6 +117,12 @@ func (s *testTypeConvertSuite) TestConvertType(c *C) {
 	v, err = Convert("12345", ft)
 	c.Assert(err, IsNil)
 	c.Assert(v, Equals, "123")
+	ft = NewFieldType(mysql.TypeString)
+	ft.Flen = 3
+	ft.Charset = charset.CharsetBin
+	v, err = Convert("12345", ft)
+	c.Assert(err, IsNil)
+	c.Assert(v, DeepEquals, []byte("123"))
 
 	// For TypeDuration
 	ft = NewFieldType(mysql.TypeDuration)
@@ -146,6 +158,11 @@ func (s *testTypeConvertSuite) TestConvertType(c *C) {
 	v, err = Convert("100", ft)
 	c.Assert(err, IsNil)
 	c.Assert(v, Equals, int64(100))
+	ft = NewFieldType(mysql.TypeLonglong)
+	ft.Flag |= mysql.UnsignedFlag
+	v, err = Convert("100", ft)
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, uint64(100))
 
 	// For TypeNewDecimal
 	ft = NewFieldType(mysql.TypeNewDecimal)