The most important potential application for VO2 is "smart windows". Producing polymer-VO2 composite film is a practicable alternative for PVD film for windows retrofitting. Here the main work should be concentrated on preparing functional filler-VO2. In this paper, high quality single crystal VO2 nanopowder and the composite film coated with this powder had been prepared by a novel and facile route. A new vanadium precursor-V2O5 center dot nH(2)O sol was fabricated by the dissolution reaction of V2O5 in H2O2. Adopting this sol, VO2 powder with an average size less than 20 nm and narrower size distribution was obtained by pre-reduction and hydrothermal treatment. The as-prepared powder showed excellent optical properties in the composite film. The films reached a sufficiently high luminous transmittance (45-60%) and keeping thermochromic effect in infrared area very well-an abrupt transmittance change greater than 50% with the phase transition was observed. Its optical properties are comparable with high quality single layer VO2 film deposited by PVD method. The as-prepared nanopowder has a phase transition temperature (tau(c)) about 55.5 degrees C and it can be tuned down to ambient by slight tungsten doping without deteriorating the thermochromic properties, which made this powder very suitable for practical application. Based on our experiments, the tau(c) reduction efficiency of tungsten was about -21.96 K/at.%w in the doping range from 0 to 2.5 at.%w. (C) 2011 Elsevier B.V. All rights reserved.