Another Method
From Parent page
protected void Button1_Click(object sender, EventArgs e)
{
string s_popScript = "<script>window.open('PopUp.aspx', null,'width=950,height=420,left=350,top=250,directories=no,titlebar=no,toolbar=no,location=no,status=no,menubar=no,scrollbars=no,resizable=no'); </script>";
Page.ClientScript.RegisterStartupScript(this.GetType(), "controlJSScript", s_popScript, false);
}