In the present work using V2O5 and MoO3 powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo6+ doped vanadium dioxide (VO2) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO2, the structure of MoO3 in the films didn't change, and the phase transition temperature of the VO2 was obviously lowered with increasing MoO3 doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO3 concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO2 doped films formed local energy level, and then reduced the forbidden band gap of VO2 as the donor defect changing its optical and electrical properties and lowering the phase transition temperature. (C) 2004 Kluwer Academic Publishers.