Molecular beam epitaxially grown Fe on GaAs(001) capped by Au overlayer with variable thickness had been investigated in situ by magnetooptical Kerr effect (MOKE). It is observed that the MOKE intensity as a function of Au film thickness decays much faster than that derived from the well-known Zak's theory. This can be explained by different optical parameters of ultrathin films from bulk values. It is also found that a minimum thickness of 1.4 nm Au is needed to effectively prevent the oxidation of Fe film at 300 K in atmosphere. (C) 2007 Elsevier B.V. All rights reserved.