Abstract: A new electrochromic device is developed by tungsten trioxide (WO3) thin film integrated with a-Si1−XGeX:H pin photodetector. With the addition of the gold (Au) film to ionize nitrogen monoxide (NO) gas, the WO3 thin film will react with ion and transfer from transparency to blue color. This color change will degrade the absorption of light with wavelength larger than blue color. Therefore, the photocurrent generated by a-Si1−XGeX:H pin photodetector will be lowered down, thus detecting the existing of NO gas. In the case of 150-ppm NO gas and 300 K operation, the induced photocurrent, the response and recovery times for the sensitivity are about 0.5 mA, 10 s, and 2 min, respectively. Especially, the WO3-pin NO sensor also shows highly selectivity with NO gas to separate from CO and C2H5OH gases.