Tungsten oxide (WO3-x) thin films for an electrochromic (EC) device were deposited at an oxygen flow ratio [O-2/(Ar + O-2)] of 0.7 using reactive facing-target sputtering with a variable working pressure. The correlation between the WO3-x thin films and EC properties was investigated. The films structural properties were measured by X-ray diffraction; the indium tin oxide diffraction peak was observed in all the films. The electrochemical and optical properties were measured by cyclic voltammetry and UV/Vis spectrometry. The WO3-x thin film obtained at 0.13 Pa displayed a maximum coloration efficiency of 31.42cm(2)/C, which indicated superior EC properties.