ok, thank you for reply me. This problem i already sovled.
But i also find a speical problem. Why I can't pass the sample cotrol text. I want to pass the text form MainForm to OptionForm.
Have you this experience? (i use vb.net mdi form, datagirdview , databinding)

                     'MainForm Button event is no problem, variable a store "1"
    Private Sub BtnBatch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnBatch.Click

      Dim a as string =""
        a = ComboBoxUsername.text              
        OptionForm.Show()


    End Sub

-----------------------------------------------------------------------------------------------------------------------------------------------
    Private Sub OptionForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim b As String = ""
        b = MainForm.ComboBoxUsername.Text           'My problem in this , b  store " " ,no data

end sub