The structural phase transition (SPT) of VO2 films epitaxially grown on c-Al2O3 substrates by pulsed laser deposition and reactive sputtering was investigated by in situ temperature-controlled x-ray diffraction (XRD) across metal-insulator transition (MIT). An intermediate insulator phase was observed between room temperature monoclinic phase and high temperature metallic tetragonal phase. Gaussian curve fittings for measured XRD patterns revealed a significant contribution of the intermediate phase covering wide temperature range across MIT. Polycrystalline VO2 films grown on Si substrates revealed pure rutile phase after MIT in contrast with the epitaxial films on c-Al2O3 substrates. Strained structure of the VO2 films grown on c-Al2O3 substrates could be a mechanism for the formation of such intermediate phase known as monoclinic M2 phase. Presence of the intermediate insulator phase in highly oriented VO2 films may be an important factor in understanding the relation between MIT and SPT.