The single crystalline Tm2O3 films are deposited on Si(001) substrates by molecular beam epitaxy, by using x-ray photoelectron spectroscopy, the valence and the conduction-band shifts of Tm2O3 to Si are obtained to be 3.1 0.2 eV and 1.9 0.3 eV, respectively. The energy gap of Er2O3 is determined to be 6.1 0.2 eV. The results of the study show that the Tm2O3 could be a promising candidate for high-k gate dielectrics.