Heteroepitaxial Y2O3 films were grown on oxidized Si(lll) and Si(100) substrates by ion assisted evaporation in ultrahigh vacuum. Film qualities such as the crystallinity, the film strain, and the morphological characteristics were investigated using x-ray scattering methods. The crystallinity of the films was investigated by reflection of high energy electron diffraction and x-ray diffraction. The morphological characteristics of the surface and interface were investigated by x-ray reflection and atomic force microscopy. From extraction of the interface and surface characteristics with the film thickness, the structural characteristics of the Y2O3 films grown on the oxidized Si(lll) showed that a mosaic structure is formed at the interfacial area. The mosaic structure was minimized as the film thickness was increased. The morphological coherences between film and substrate, the strain change, and the sharp diffraction peak showed that the film surface of Y2O3(111) grown on a mosaic structure had a perfect crystalline structure and flat surface. This crystalline structure changed into a grain growth structure as the film thickness increased. However, the changes in the morphological coherence, the crystallinity, and the strain of the growth system of Y2O3(110)/Si(100) with the film thickness suggest that the film was grown with grain like islands in all the thickness ranges.