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 is thus necessary. The efficiency of traditional manual testing methods can not meet the production requirements. An automatic visual inspection system for gas valve’s external screw taper thread is presented in the paper. The system consists of a high-performance 200 mega pixels 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 half of thread angle, depth of thread, taper angle of thread on image domain. Due to the lead angle and installation error, actual half of thread angle is different from the theoretical values. Three methods are presented in the paper for the half of thread angle. Through analysis, Method 3 and Method 2 can be used to measure half of thread angle. The system can meet the testing requirements of the external screw taper thread and has wide application perspective.