Image-based on-line monitoring system is widely applied in China recently, of which the image recognition is one of the key techniques. Considering the image feature and contour characteristic of the transmission line and such accessories as the damper, insulator chains, a recognition method based on chain code tracking and corner point detection is proposed. First extract the edge contour of the components after the image preprocessing, including graying, linear grey transforming upon the original image. Then trace chain code and detect corner points of the edge contour. Finally extract circle contour and determine its center point for the damper end face. Recognition experiment performed on fifty images proves that the proposed method is effective for detecting almost all corner points on image contour and recognizing the objects with 99% reliability.