Tm 2 O 3 crystalline films have been deposited on Si (001) by molecular beam epitaxy (MBE). Band alignments of Tm 2 O 3 /Si gate stacks were studied by X-ray photoelectron spectroscopy (XPS). According to XPS measurements, it can be noted that a valence-band offset of −3.1±0.1eV and a conduction-band offset of 2.3±0.3eV for the Tm 2 O 3 /Si heterojunction have been obtained. Based on analysis from O1s energy-loss spectrum, the energy gap of Tm 2 O 3 is determined to be 6.5±0.3eV. A relatively thicker interfacial SiO x layer was observed for the as-annealed samples. However, no apparent change in band alignment has been observed for Tm 2 O 3 /Si heterojunction with the formation of interface layer, which has been discussed in detail.