Synchrotron radiation photoemission and ferromagnetic resonace(FMR) measurement have been used to study the interface formation of Co/GaAs(100) as well as the magnetic property of ultrathin Co films.The results show strong interface disruption and reaction between overlayer and substrate at low Co coverage(～0.2nm),At the coverage of 1nm,a stable interface forms.The Ga atoms in bulk GaAs may exchange with Co atoms and diffuse into Co overlayer,while a certain amount of As reacts with Co atoms,forming stable Co-As bonding.These reaction products lie in the narrow region near the interface(0.3—0.4nm).The remanent part of As will segregates on the surface of Co overlayer.Based on a theoretical model,the interface structure and concentration profile are discussed in detail.The FMR result demontrates that the ultrathin Co film consists of good-quality crystals and has a chemical homogeneity.