The effects of the Si surface state on epitaxial growth of Y2O3 layers were investigated by various measurement methods. The characterization using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS) and high-resolution transmission electron microscopy (HRTEM) shows excellent film crystallinity when grown on an oxidized Si surface. The crystalline structure of the film was influenced by the interfacial mosaic structure, which depended on whether the Si surface contained adsorbed O or not. The thin SiO2 layer of approximately 1.5 nm, provided favorable interfacial reaction sites for the nucleation of Y2O3, and still maintained the structural registry with the underlying Si substrate. In particular, the reaction between the Y and SiO2 layer resulted in coherent finite growth, whereas the direct interaction between Y and Si was hindered by the nucleation of Y2O3. The high-quality epitaxial layer with the minimum channel yield (chi(min)), lower than 3%, could be grown on the oxidized Si surface.