Abstract: The quantitative detection of the workpiece surface defect has been mentioned rarely. In order to solve this problem, a measuring system which consists of a collimated laser diode, a camera, a microscope and a translation stage is built. The laser is chosen as the light source, and the light pattern on the surface is picked up by the camera with a microscope. The images of light pattern are processed by using centre of gravity method, and the width of the surface defect can be obtained by analyzing the pixels distribution corresponding to the defect. Some experiments have been done, and the results show that this system can detect the defect position and calculate the size.