A highly oriented FeO thin film was formed from a Fe3O4 thin film containing Fe nanocrystallines by post-annealing at 600 degrees C. Fe3O4 thin films were grown on Si(100) substrates by ion beam sputter deposition under oxygen ambient. The stoichiometry of the iron oxide thin film could be precisely controlled by in situ X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) pattern of the Fe3O4 thin film grown at substrate temperature of 300 degrees C showed a mixed phase of Fe3O4 and Fe nanocrystallines with a preferred orientation (110). However, the mixed phase was converted to a highly oriented FeO(200) phase by post-annealing at 600 degrees C. This could be inverted as a result of Ostwald ripening of the Fe3O4 and Fe nanocrystallines.