We investigated the dependence of the Y2O3 film growth on Si surface at initial growth stage. The reflection high-energy electron diffraction, X-ray scattering, and atomic force microscopy showed that the film crystallinity and morphology strongly depended on whether Si surface contained O or not. In particular, the films grown on oxidized surfaces revealed significant improvement in crystallinity and surface smoothness. A well-ordered atomic structure of Y2O3 film was formed on 1.5 nm thick SiO2 layer with the surface and interfacial roughness markedly enhanced, compared with the him grown on the clean Si surfaces. The epitaxial him on the oxidized Si surface exhibited extremely small mosaic structures at interface, while the film on the clean Si surface displayed an island-like growth with large mosaic structures. The nucleation sites for Y2O3 were provided by the reaction between SiO2 and Y at the initial growth stage. The SiO2 layer known to hinder crystal growth is found to enhance the nucleation of Y2O3, and provides a stable buffer layer against the silicide formation. Thus, the formation of the initial SiO2 layer is the key to the high-quality epitaxial growth of Y2O3 on Si.