The failure of the gas valve’s external screw taper thread is a very serious problem which may result in accidents. Routine inspection on the threads are thus necessary. The efficiency of traditional manual testing methods can not meet the production requirements. An automatic visual inspection system for gas valve’s exteranl screw taper thread was presented in the paper. The system consists of a high-performance 200 megapixels CCD camera, a LED backlight and a holder of gas valve. Through image acquisition, image processing, edge detection and feature dimension measurement, the system can effectively measure the parameters of taper thread such as angle of thread, depth of thread, taper angle of thread on image domain. It can meet the testing requirements of the external screw taper thread and has wide application perspective.