A spur gear tooth number and missing teeth identification algorithm based on minimum convex hull and convex defects of outer contour is presented in the paper. Firstly, the connected domain of binary image is adopted to trace the outer contour of gear. Then, the minimum convex hull and convex defects of outline are detected, and fake convex defects need to be removed before identifying tooth number and missing teeth part. According to the shape characteristics of spur gear, an addendum circle detection method based on the convex hull vertices of gear outer contour and a dedendum circle fitting method based on the deepest point of convex defects are proposed, which have realized the rapid measurement of characteristic circle parameters of spur gear. The experimental results show that the accuracy of the proposed tooth number and missing teeth recognition algorithm is as high as 98.48%. Compared with the existing two kinds of typical methods, the proposed characteristic gear circle fitting method can achieve high speed and high precision positioning of addendum circle and dedendum circle under various conditions.