In this work, we propose a cooperative server-client HTTP adaptive streaming system to provide a high-quality live video streaming service. In the proposed system, the server adaptively encodes the video segment to improve the bandwidth utilization of clients according to the bandwidth information collected from clients. Additionally, the client selects the segment bitrate by considering the bandwidth utilization, quality difference, and buffered playback time. The performance of the proposed system is verified through a simulation. The simulation results show that the proposed system can provide higher bandwidth utilization and lower quality difference than existing HTTP adaptive streaming systems.