The defect structures of vanadium dioxide (VO2) films prepared at different oxygen pressures have been investigated using positron annihilation spectroscopy for the first time. It is found that the concentration of vanadium vacancies is not dependent on oxygen pressure for the range studied, implying that at high oxygen pressure, the point defects which have an effect on transition properties are O-interstitials. The variations of oxygen pressures are more probable to cause changes of the type or concentration of oxygen related defects (such as O-interstitials or O-vacancies) and further influence the transition characters. No matter what kind of the defects they are, the localized point defects have a great impact on the metal-insulator phase transition (MIT) process and the corresponding influence mechanisms are discussed. In addition, the stability of the VO2 films is also studied. The present results are valuable for the achievement of VO2-based devices. (C) 2014 Published by Elsevier B.V.