Thank you for reply me first ...........
But i already try your method , but also can't leave the MaskedTextbox, No error msg.........
In database, dataset i already set the Quodate field allow null value YES...........

Mycode:

MskQuodate.DataBindings.Add("text", QuohdrBindingSource, "Quodate", True, DataSourceUpdateMode.OnPropertyChanged)

  Private Sub MskQuodate_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MskQuodate.Validating
        If (MskQuodate.Text.Substring(0, 3) = "  /") Then

            MskQuodate.Text = ""
        End If
    End Sub

So, i don't know how can i do............ But i search the website, i find the same problem........
http://connect.microsoft.com/VisualStudio/feedback/details/116519/cant-delete-values-in-masked-text-box

But i saw this , i also don't know to do............ Any one can give me idea? thx~~~