In this Paper the superparamagnetism of the oxidzed surface layer of fine iron paricles is reported. Fine iron particles in the range of size from 15 to 50nm with oxidized surface ayer were prepared by evaporation in nitrogen atmosphere. The sizes were measured by the BET method and transmission electron microscope (TEM). X-ray and electron diffraction analysis show that the particle is coposed of α-Fe core and a sufrface layer of Fe3O4. The thickness of the outer oxidized layer is about 2nm and is independent of the average diameter of the particles. The results indicate that the transition of the domain structure plays an important role in the superparamagnetic behavior of the oxide surface layer of iron particles. The emergence of the superparamag-netism at low temperature, may be a diagnostic of the transttion from the smgle domain to multi-domain structre for inner core. The criticle size of the single domain domain is esatimated at-about 40nm.