Abstract: Inview of the problem of dimension measurement of circular mechanical parts such as bearing inner and outer circle, combined with the image processing module of MATLAB, in order to improve the precision and speed of detection for the principle, the aperture diameter automatic detection system based on image recognition was designed. The outer diameter and inner diameter of the bearing were accurately extracted by image preprocessing and edge detection of the collected workpiece images, and the outer diameter and inner diameter of the bearing were calculated by using the least square method to fit the edge points round. The experimental data show that the system meets the requirements of measurement accuracy and has high accuracy, solves the problems such as low efficiency and waste of labor in traditional manual detection, and provides a useful reference for automatic dimension detection of mechanical parts.