Motion-detection and rate-control are both important modules in the video-encoding system for surveillance. Conventional video encoder needs large amount of extra memory and circuit to implement the motion-detection function, and has a large computational complexity on rate-control function. To solve the aforementioned problems, this thesis proposed a rate-control algorithm based on motion-detection. Based on conventional video encoder architecture, the algorithm applied motion-detection function with fewer hardware cost, applied complexity-analysis and rate-control function based on the motion-detection result. The experiment result approved that the algorithm reached desired motion-detection result and rate-control result while with lower hardware cost and with a satisfying compatibility with various of video encoder standards.