Abstract: Aiming at a conveyer line of heated ingots in a roll mill, a method is presented for online measurement of workpiece length. The image information of a workpiece on a production line is acquired by a CCD camera at distance, and then the edge noise of the image is filtered. The real-time measurement is carried out using a simple algorithm or the gradient operator of image grey scale to detect and recognize quickly the edge of an image, thus acquiring the workpiece length. The quadratic polynomial is fitted to compensate the non-linearity of the camera lens so as to improve the measuring precision. Introduces the structure and functions of a real-time system for online measurement and analyzes its speed and precision. The results show that when the workpiece length is 400-1200 mm with conveying speed less than 5 m/s, the measuring error is lower than 0.4%.