In this study, tin-oxide (SnO2) thin films were prepared by a two-step process. With a vacuum-evaporation method, pure tin thin films were first deposited on Si and glass substrates at substrate temperatures from 50 to 300 degrees C. Then the metallic thin films were thermally oxidized with dry oxygen from 300 to 500 degrees C. The surface morphology of the SnO2 thin films was observed by scanning electron microscopy and atomic force microscopy. It was found that metallic hillocks are formed only for SnO2 thin films for which the substrate temperature during tin deposition was lower than 150 degrees C. The hillock growth is associated with stress inhomogeneity during the oxidation process. Grain size of the SnO2 thin films is also found to increase with the film thickness and the oxidation temperature. Kinetics of the grain growth is discussed in terms of a three-dimensional diffusion limited process.