Hi Bahare,
whats not possible you cannot find button in your control..because i tried the same and it worked:
protected void OnClick(object sender, EventArgs e) //function on page
{
Button btn = (Button)MyContol.FindControl("Button1"); //Button1 is in control, NyControl is placed in this aspx page
}
or is it state of the Button that you cant find.. let me know.