A series of WO3 films are prepared by rf-sputtering under different partial pressures of oxygen. It is found that the films with oxygen partial pressure ratio 1∶10 have better electrochronic(EC) properties than others. Microstructural and compositional characteristics of WO3 films are studied by x-ray diffraction, atomic force microscopy and x-ray photo electro spectroscopy. It is obvious that the samples are amorphous and have more pores that are beneficial for Li+ to inject and eject. The original-state films are composed of tungsten and oxygen. After electrochemical reactions， the main component becomes LixWO3 in which tungsten ions normally exhibit both W6+ and W5+ oxidation states. The films are annealed in air at 230 and 400℃ for 90 min. The cyclic voltammograms and optical transmittance show that the films annealed at 400℃ after more than 10 cycles present better EC properties.