An on-line image measurement system for curve grinding was schemed out according to the working process. Because of interaction between detection precision and field of view, it is difficult to realize high detection precision at a large field of view. In order to settle this problem, a detection method based on circular tolerance zone was presented according to grinding process and wheel shape. Real-time images of work piece can be gathered while using synchronal control and outer trigger technology. Using curve fitting method, the work piece image edge can be located to sub-pixel values. Experiments show that the proposed method in this paper is effective, and its detection precision and results are reasonable.