Single-crystal Al2O3 films have been epitaxially grown on Si (111) substrates despite a lattice mismatch of more than 30%. The oxide was electron-beam evaporated from a high-purity sapphire source. The structural and morphological studies carried out by x-ray diffraction, x-ray reflectivity, atomic force microscopy, and transmission electron microscopy, with the initial epitaxial growth observed by in situ reflection high-energy electron diffraction show that the oxide films as thin as 3.8 nm have the cubic γ-phase with a very uniform thickness and a high structural perfection. The film surface is very smooth with a roughness of 0.12 nm and the oxide/Si interface is atomically sharp. The γ-Al2O3 films are well aligned with Si substrate with an orientation relationship of Si(111)//Al2O3(222), Si//Al2O3.