VO2 films with different thicknesses were deposited on SiO2 substrate by an inorganic sol-gel method. The variation of stoichiometry, crystalline structure and surface morphology of the films with thicknesses were analyzed by X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy, respectively. Furthermore, the effects of the composition and microstructure on the phase transition property of the films were studied in the mid-infrared range. The results indicate that all the films exhibit a (110) preferred orientation; the crystallinity is enhanced, and the grain size is improved with broader distribution in the thicker films. Accordingly, the thicker films show reduced transmission before and after the phase transition, narrower hysteresis loop and larger transition slope.