The composition depth profiles of the Mn thin films grown on GaAs(00l) surface using MBE technique are studied with X-ray photoelectron spectroscopy(XPS) . The experimental results show that the fcc-Mn/GaAs(001) system grown on a 400 K substrate has a sandwich structure with a Mn-Ga-As buffer layer located between the fcc-Mn layer and the GaAs substrate; the α-Mn/GaAs (001) system grown on a 300 K substrate also has a similar buffer layer which is much thinner than that of the fcc-Mn/GaAs(001) system;and the system grown on a 450 K substrate is a Mn-Ga-As alloy beyond the GaAs substrate and it has no Mn-dominated area. It is concluded that to keep the substrate at a proper temperature(400 K) during growth to obtain a suitable thickness of buffer layer is an essential requirement to grow fcc-Mn on GaAs(00l) surface.