To measure the sub-pixel image motion caused by satellite attitude instability or various disturbance, an effective approach to measure image motion by dual-CCD imaging system based on joint transform correlator (JTC) is presented. An auxiliary high-speed CCD is used to capture image sequences which are the same scenes as those images captured by the prime CCD. These image sequences are optically calculated by JTC system, and then relatively motion displacement can be obtained. The principle of this approach is represented, and measurement precision is analyzed in the condition of noise and image motion. Motion measurement system is also presented, and the experimental results show that the measurement precision can be controlled within 0.1 pixel.