try this,put the line in ur button click event
ScriptManager.RegisterClientScriptBlock(this, typeof(string), "OPEN_WINDOW", "var Mleft = (screen.width/2)-(760/2);var Mtop = (screen.height/2)-(700/2);window.open( 'MyPage.aspx?recepitno=" + ID + "', null, 'height=600,width=650,status=yes,toolbar=no,scrollbars=yes,menubar=no,location=no,top=\'+Mtop+\', left=\'+Mleft+\'' );", true);