The electronic structure of b-axis oriented VO2 thin films in monoclinic phase with various thicknesses has been studied by soft-X-ray spectroscopy. The VO2 thin films show the mixed-valence V3+/V4+ state, which is created by oxygen vacancies. The V3+ state and oxygen vacancies increase with increasing film thickness. The thinnest film of 56 nm exhibits a metal-insulator transition (MIT) at ∼340 K, although thicker films of 126 and 155 nm do not exhibit MIT. The valence band structure around the Fermi level consists of V 3d e(Formula presented.), a1g, and e(Formula presented.) bands. The bandwidths and spectral weights of the a1g and e(Formula presented.) bands are smaller in the thinnest film, which corresponds to the insulating phase at 300 K. The above results indicate that the mixed-valence states with oxygen vacancies are closely related to the MIT in the monoclinic phase of VO2 thin films.