Epitaxial gamma -Al2O3 films were grown on a chemically oxidized Si(111) substrate by ionized beam deposition. The effects of an oxidized Si surface on the film properties were examined and compared with the results of the Al2O3 films grown on a clean Si surface. Al2O3 films grown on an oxidized Si surface showed higher crystalline quality, a flatter surface, and a more abrupt interface than those of the films grown on a clean Si substrate. Temperature dependence of Al2O3 films on the crystallinity and surface morphology was estimated by reflection high-energy electron diffraction and atomic force microscopy. Chemical composition and interface state of the films were evaluated by X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy.