No Sir, I didnot get any error message in message box.
If i had got such error message, then it would have been corrected by me.

Catch ex As Exception
MsgBox(ex.ToString)
End Try

I have putted this code in the button_click event.
and i have also applies try catch statement too. but that i did not copied here.
But still the error was not shown. :-(

Thanks Again Charlie.
I made a very stupid mistake.
The problem was with the order of parameters as said by you earlier.

now i have put my connection string into the condition.
but now also the same problem is occuring.

There is no error , program terminates after successful completion but database is not updated.My new code is:

Dim dbPath As String = "D:\WindowsApplication1\Medical.mdb"
                con.Close()                con.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;" _
                & "Data Source=" & dbPath & ";Jet OLEDB:Database Password=guptaji;"

                Dim bal As Integer = Val(Me.TextBox2.Text) - Val(Me.TextBox7.Text)
                MsgBox(bal)
                str = ("UPDATE Add1 SET [Quantity]=@Quantity where [PName] =@PName")
                Dim acscmdup1 As New OleDb.OleDbCommand()
                acscmdup1.CommandText = str
                If (con.State = ConnectionState.Closed) Then
                    con.Open()
                End If
                acscmdup1.Connection = con
                With acscmdup1
                    .Parameters.AddWithValue("@PName", Me.ComboBox3.Text)
                    .Parameters.AddWithValue("@Quantity", bal)

                    .ExecuteNonQuery() 'Execute command
                End With
                acscmdup1.Dispose()
                MessageBox.Show("Database Updated", "Updated..", MessageBoxButtons.OK, MessageBoxIcon.Information)
                con.Close()

Help