Al2O3 thin films were epitaxially grown on Si(001) substrates by using reactive ionized beam deposition. The substrate temperature dependence of crystallinity of Al2O3 films were investigated by in situ reflection high-energy electron diffraction. The epitaxial gamma -Al2O3 stacked structure was formed at above 850 degreesC, and below this temperature the films were polycrystal. Film compositions and interface states were investigated by x-ray photoelectron spectroscopy and transmission electron microscopy measurements. From these results, stoichiometric Al2O3 films with sharp interface were confirmed. The rms surface roughness of the epitaxially grown Al2O3 film was 0.46 nm, and as the substrate temperatures increased, the crystalline quality and the surface flatness were improved.