In this work, we investigate the electrical and optical properties of an electrochromic device (ECD) that has molybdenum-doped tungsten trioxide (WO 3 :Mo) electrode. Also, it's surface morphology, coloration efficiency and light transmittance are evaluated. To fabricate the ECD, the WO 3 :Mo film as the working electrode and the Nickel oxide (NiO) film as the counter electrode are prepared by RF magnetron co-sputtering deposition respectively. WO 3 and Mo are set as targets and fluorine-doped tin oxide (FTO) glass, size 2 × 2 cm, is prepared as a substrate. Both targets are sputtered under the same sputtering conditions such as gas pressure, target distance and rotational speed of substrate at 15 mTorr, 5 cm and 5 rpm, respectively. In the co-sputtering process, all conditions were the same for the WO 3 target and Mo target, but only the RF power applied to each target was changed. The RF power applied to the WO 3 target was fixed at 150 W, while the RF power to the Mo target was increased from 50 W to 250 W in 100 W increments giving a difference in dopant concentrations. At the RF power to the Mo target of 50 W, 150 W, and 250 W, respectively, the coloration efficiency of the ECDs was 67.58 cm 2 C −1 , 40.60 cm 2 C −1 , 36.89 cm 2 C −1 . The optimum RF power to the Mo target for the fabrication of an WO 3 :Mo film for use as an electrode in ECD was 50 W at the fixed 150 W RF power to the WO 3 target, and the fabricated films showed an amorphous structure and the highest coloration efficiency. The transmittances of the prepared ECD were 64.63% and 17.17%, respectively, in the bleaching and coloring state. • RF Magnetron co-sputtering was used for fabricate the WO 3 :Mo thin film for made the electrochromic devices. • The concentration ratio was adjusted by varies the RF power applied to Mo target. • The optimal parameter was obtained from RF power was 150 W to WO 3 target and 50 W to Mo target. • The CE of the ECD with WO 3 :Mo electrode were increased to 67.58 cm 2 ·C -1 compare to undoped WO 3 electrode 41.45cm 2 ·C -1 .