The growth kinetics of the oxide film and oxidation mechanism on tin films prepared by the electron-beam evaporation in the temperature range of 250—400℃ by an isothermal process were investigated. Based on an X-ray diffraction, auger electron spectrum, scanning electron microscope and alpha-step instrument, the evolution of the structure, composition, morphology and thickness of the oxide on tin films has been studied. In the studied temperature region, the growth of the oxide film was found to obey a parabolic growth-rate law with an activation energy of about 0.34eV, and is controlled by the oxygen diffusion from the loose oxide. It is concluded that the growth of the oxide begins from the formation of a SnO phase，with the increase of the oxidation time, the SnO phase decomposes and a Sn3O4 phase forms due to the SnO thermal and chemical unstability, and the deeper oxidation transfers the Sn3O4 phase to a SnO2 phase.