It has been found that the molar ratio of V2O5 to N2H4 was the critical factor for the preparation of VO2 nano-powder in this system, while the temperature need to be considered as another important parameter. Temperature lower than260 C leaded to a lower crystallinity of VO2 and the long period was needed for hydrothermal treatment. So in this paper, most of experiments were carried out at 260 C. The value of the molar ratio of V2O5 to N2H4 could only be varied in a very narrow range for the control of chemical valance of vanadium. In most cases, the final products could not be identified as one signal phase or any known phases when the molar ratio was greater or lower than the proper value. Fig. 2 showed three typical XRD patterns for samples fabricated with different molar ratios where the molar ratio was proper (Fig. 2(a)) or slightly departed from the proper value. It also indicated that the chemical valance of vanadium is very volatile and many non-stoichiometric substances could be prone to be formed aroundstoichiometricVO2.