Finnaly, i find the answer. i use checkboxlist control, because there is a bug in radiolbuttonlist control when RepeatColumn is set to 2
if form_load :
If Not IsPostBack Then
ViewState("Chk") = 2
CheckBoxList1.SelectedIndex = 2
End If
and .....
Protected Sub CheckBoxList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles CheckBoxList1.SelectedIndexChanged
CheckBoxList1.SelectedIndex = ViewState("Chk")
End Sub
It will push the selectedindex value to the viewstate value.