Er2O3 thin films are grown on oxidized Si (111) substrates by molecular beam epitaxy. The sample grown tinder optimized condition is characterized in its microstructure, surface morphology and thickness using grazing incidence x-ray diffraction (GIXRD), atomic force morphology and x-ray reflectivity. GIXRD measurements reveal that the Er2O3 thin film is a mosaic of single-crystal domains. The interplanar spacing d in-plane residual strain tensor ε|| and the strain relaxation degree ξ are calculated. The Poisson ratio μ obtained by conventional x-ray diffraction is in good agreement with that of the bulk Er2O3. In-plane strains in three sets of planes, i.e. (440), (404), and (044), are isotropic.