With V2O5 and oxalic acid as raw materials, VO2(B) powders were synthesized by hydrothermal method. The structure and morphology of the products were characterized by XRD, SEM, TEM and HRTEM. The growth mechanism of the VO2(B) powders was investigated by studying the effect of hydrothermal temperature and hydrothermal time on the structure and morphology of the products. The results show that, under the hydrothermal process, V2O5 first transforms into V6O13 and then reacts with oxalic acid to form the final product VO2(B), and the laminated structure VO2(B) changes into the subsequently stable VO2 (B) powders gradually, in which the length is 1μm, the wideness is 100~200 nm and the spacing of the lattice fringe is 0.51 nm with prolonging hydrothermal time.