Heteroepitaxial Y2O3 films were grown on oxidized and clean Si (100) surfaces by ion assisted evaporation under an ultrahigh vacuum. The crystalline structure, crystallinity, morphology, and electrical properties were investigated using various techniques. The crystallinity assessed by x-ray diffraction and Rutherford backscattering spectroscopy shows that the films grown on the oxidized Si substrates have better crystallinity and smoother morphology compared to those on the clean Si. As the annealing temperature increases, the crystallinity and morphology are stable for the films grown on the oxidized Si, while those of the films grown on clean Si substrates degrade. The difference between the two films is attributed to the formation of hillocks and a chemical reaction at the interface between the film and SiO2. The low crystallinity, strain change, and the reaction of excess Y in the films grown on the clean Si contribute to the crystalline structure and the formation of hillock. These changes of crystallinity and morphology show that the films grown on the oxidized Si surface are more suitable for device applications. Thus, the films grown on the oxidized Si result in higher breakdown field strength and lower trap charge density than those on clean Si after the annealing treatment.