Tungsten trioxide thin films are successfully synthesized by a sol-gel method using tungsten hexachloride as precursors. The structural, chemical, and optical properties of the prepared films are characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV-Vis spectrophotometry. The electrochemical and electrochromic properties of the films before and after heat treatment are also investigated by cyclic voltammetry, chronoamperometry, and in situ transmittance measurement system. Compared to as-prepared films, heattreated tungsten trioxide thin films exhibit a higher electrochemical reversibility of 0.81 and superior coloration efficiency of 65.7 cm²/C, which implies that heat treatment at an appropriate temperature is a crucial process in a sol-gel method for having a better electrochromic performance.