protected void ddlsupplier_SelectedIndexChanged(object sender, EventArgs e)
{
if (ddlsupplier.SelectedValue == "New")
{
Response.Write("<script language='javascript'>window.open('AddSupplier.aspx','Popup','width=550,height=370,left=350,top=200,resizable=no')</script>");
}
}
done using this code