Ferromagnetic MnAs thin films were grown on clean, S- and Se-passivated GaAs(1 0 0) substrates by molecular beam epitaxy (MBE). Growth modes of MnAs films on GaAs substrates depend on the surface treatment. On the clean and the Se-passivated surfaces MnAs films grow by a layer-by-layer mode, whereas on the S-passivated surface MnAs films grow by an island growth mode, and as a result polycrystalline films are obtained. Photoelectron spectroscopy study revealed that the Se overlayer on the Se-passivated GaAs substrates prevented Ga interdiffusion. We also found that the saturation magnetization of MnAs films on the Se-passivated surface was slightly larger than that on the clean and the S-passivated surfaces. This difference can be explained by the role of Se overlayer in preventing interdiffusion between MnAs and GaAs.