Abstract: In order to solve the problem that hole diameter of hard disk is easy to be affected by local strong reflection and external noise, and low accuracy of measurements, a method for measuring the diameter of circular parts based on wavelet transform, mathematical morphology and machine vision is proposed.Hole image is collected by camera, high/low-frequency images are resolved through wavelet transform, and wavelet transform is used to extract edge of high frequency part and mathematical morphology is used to extract low frequency part, then wavelet edge fusion is used to obtain effective hole edge.Finally, the least square method is utilized to calculate pore size. It is verified experimently that, compared with other algorithms, in the case of effectively preserving edge information of round hole and suppressing surrounding noise effectively, detection accuracy is controlled within 0.01 mm.Experimental results show that this method is simple and feasible, and has a high accuracy.