Epitaxial vanadium dioxide thin films with different thicknesses were deposited on Al2O3 (0001) substrates by using a polymer-assisted deposition technique.The metal-insulator transition of the films was investigated by the temperature dependency of Raman spectrum and Fourier transform infrared spectroscopy.It is interesting to observe that the occurrence window of the intermediate monoclinic phase (M2) is related to the thickness of the film.By carefully studying the microstructures of the as-prepared thin films by high resolution X-ray diffraction, it is suggested that this is associated with the nucleation of the new phase at the domain boundaries, which is strongly affected by the epitaxial quality.