Aimed at the problem of inspecting the appearance defects of ring, a vision detection algorithm based on polar coordinate transformation was proposed on the basis of the use of vision technology. The problem of tipes of the appearance defects of ring was analyzed, polar coordinate transformation and interpolation algorithms were used to convert the ring region into the rectangular region. Subpixel edge of ring was extracted to measure the ring width in the rectangular region, the upper and lower edge coordinate array was got in ring rectangular area and the average value of their longitudinal coordinates was calculated to search for suspicious defects. Finally the maximum and minimum width of the ring was calculated and compared with the standard ring width to determine the type of defect,then the position of defects was judged according to the longitudinal coordinates of the defect. The software of Halcon was used to implement the algorithm, the expriment shows that the proposed algorithm can accurately measure ring width and inspect the ring appearance defects