When an object makes a motion of continuous variation, its projection on a plane brings a succession of image and the motion between the video camera and the object causes displacement of image pixels. The relative motion of the displacement is called optical flow. The advantage of using the optical flow approach is that it is not required to know characteristics of the object and the environment at that time. So this method is suitable for tracking problems in unknown environment. It has been indicated that the optical flow based on the whole image cannot always be correct enough for control purpose where motion or feature occur. This thesis first uses digital image technique to subtract two continuous images, and extract the region where the motion actually occurs. Then, optical flow is calculated based on image information in this area. In this way, it cannot only raise the tracking speed, but also reduce the effect of the incorrect optical flow value. As a result, both tracking accuracy and speed can be greatly improved.