We investigated the dependence of the crystallinity, strain, and morphological characteristics of epitaxial Y2O3 films grown on Si(lll) by ion beam-assisted deposition. Various characterization tools, such as reflection high-energy electron diffraction, X-ray diffraction, high-resolution transmission electron and atomic force microscopy, were used to reveal the physical properties which depend on the assisted energy of the oxygen-ion beam. When the assisted energy of the oxygen-ion beam was applied to grow films, the growth temperature for epitaxy was lowered. The crystallinity was improved and the film was compressed as the assisted energy increased up to 45 eV, while the improvement of crystallinity and the strain increment was suppressed as the assisted energy increased further. Moreover, the morphological shape shows that island growth is induced when ion energy is supplied. That is, when the ion energy is increased, islands are expanded and the surfaces are flattened. The surface morphology yields information on film characteristics. such as crystallinity and strain, both of which depend on the assisted energy of the oxygen ion.