ok, thank you for reply me. This problem i already seen you blog and sovled it.
But i also find a speical problem. I don't know , i can't pass the sample cotrol text. I want to pass the text form MainForm to OptionForm.
And then i think this problem , it lead why i can't to use DataGridView.SelectedRows method to detect and get the selected rows.
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

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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