Tungsten oxide thin films were prepared by depositing WO3 onto glass substrates coated with ITO using reactive evaporation process at ambient temperature and 200°C respectively. The thin films were grown at different deposition rate. Chronoamperometry was carried out and spectral measurements were performed in situ. Results showed that the thin films prepared at low deposition rates possess higher coloration efficiency (CE), and the thin films grown at ambient temperature have high CE than those grown at 200°C. The origin of the differences in coloration efficiency of the thin films were analyzed and discussed based on the electrochromic mechanism of amorphous tungsten oxide films. The samples morphology was characterized by atom force microscopy (AFM).© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.