The phenomenon of color change in a particular material caused by an electrochemical reaction is called "electrochromism." Since the discovery of the electrochromic phenomenon in tungsten oxide, amorphous WO3 is the most widely studied electrochromic material. Until now, WO3 remains the most promising candidate for electrochromic devices. Electrochromic devices can switch between two optical states (colored/bleached) by injection/extraction of ions and electrons, based on the modification of transmittance and reflectance that are induced by an applied external voltage. In this study, tungsten oxide (WO3-x) thin films were deposited onto an ITO-coated glass substrate by reactive facing-target sputtering, and their electrochromic properties studied as a function of film thickness. The X-ray diffraction patterns reveal the amorphous properties of all the WO3-x thin films prepared in this study. Of all the different samples prepared in this study, WO3-x thin film of thickness 500nm exhibited the maximum coloration efficiency of 37.3cm(2)/C.