Ultrathin iron films have been widely adopted in the high-density magneto-optical recording area.[1-6]In magneto optical Kerr effect (MOKE) measurement,the optical constants and film thicknesses of ultrathin iron films are of great importance.They must be measured accurately to obtain accurate magneto optical Kerr rotation results.[7,8] For ultrathin iron films,the optical constants are obviously different for bulk materials and thick films.The measurements of optical constants and film thickness of metal films are generally carried out by using ellipsometry.