Abstract: A new electrochromic device (ECD) is developed by tungsten oxide (WO3) thin film integrated with a-Si1−XGeX:H pin photodetector. With the addition of the palladium (Pd) film to ionize hydrogen gas, the WO3 thin film will react with hydrogen ion and transfer from transparency to blue color. This color change will degrade the absorption of light with a wavelength larger than the blue color. Therefore, the photocurrent generated by a-Si1−XGeX:H pin photodetector will be lowered down, thus detecting the existing of hydrogen gas. Especially, the WO3-pin hydrogen sensor also shows highly selectivity with hydrogen gas to separate from CO and C2H5OH gases.