Abstract: The icing thickness measuring principle of transmission lines is discussed based on image processing. A new method of measuring icing thickness is proposed for transmission lines to overcome the limitation of current technologies. The image of transmission line is real-time processed by this method, and the icing image is disposed by using of median filtering, which removes the noise better than other filter method and avoids the fuzzy boundary. The actual image edge can be identified as much as possible in edge detecting by using Canny algorithm. Improved Hough transform method is blended in classifier and equal matrice-matching mechanism, which reduces the storage of Hough transform, and improves the reliability of the Hough transform and the accuracy of the algorithm. The icing thickness of transmission line is measured by the extracting of edge contour image. The experimental results show that the method improves the accuracy of extracting contour image and has certain application prospect.