Electrochromic thin film devices are in advanced stage of development for optical application. Films of Tungsten oxide were deposited in the vacuum chamber and were treated at different temperature. Properties of films were studied. The color of the fresh film is closely related with the substrate temperature, changing from colorless to light yellow and even to light blue when the temperature of substrate increased. Infrared spectra results indicate that proton exists in the film as a form of O-H, and that the strength of absorbed peak of active O-H at 3300-3450/cm increases when the substrate temperature rises. The strength of peak of O-H vibration depend on the substrate temperature. No absorbed peak of H-O-H vibration appears. Measurement of visible spectra of the electrochromic film of a-WO3 was done for different conditions of the electrochromic process. The visible light transmittance of bleached film, which stays the same in the electrochromic cycle, was always different from the fresh film.