Ultrathin Fe film 200  V/4  Fe/900  V/MgO（100） has been prepared by molecular beam epitaxy （MBE）. The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity measurement. The magnetic properties of the thin Fe layer were investigated by polarized neutron reflectometry at different temperature. The result shows that the magnetic moment of an Fe atom is about 1.0±0.1　μB at room temperature and increases to 1.5±0.1 μB at 10　K. The Curie temperature of the thin Fe film is estimated to be 310±30K.