Thin tin films were deposited on oxidized silicon substrates by sputtering. Changes in both the phase and morphology of the films were studied as a function of the annealing process, time and atmosphere. X-ray diffraction (XRD) analysis was used to study phase evolution of the films during annealing. Atomic force microscopy and scanning electron microscopy were employed to characterize microstructure and morphology of the films. XRD analyses revealed phase transformations from metallic Sn to romarchite and finally to cassiterite as a function of higher annealing temperature. Differences in the phase transformation rate from metallic tin films to tin dioxide were observed between oxygen and air atmospheres. Little variation in grain size was observed during oxidation annealing from metallic to oxide states in either air or oxygen. However, surface roughness of the films was strongly dependent on the annealing atmospheres. The films annealed under air showed that surface roughness increased linearly with time and temperature, while the surface roughness change of the tin oxide films annealed under oxygen exhibited saturation behavior. The surface morphology change of the tin oxide films processed in oxygen exhibited different behavior from those processed in air.