For single-antenna video communications, the potential unreliability of direct link and the delay constraints of video are two primary factors that influence the reconstructed video quality. A novel cooperative transmission scheme is thus proposed to tackle this problem. Specifically, the source and relay jointly retransmit the corrupted packets by suitable Space-time block codes (STBCs), which raises the capacity utilization of random channel. Additionally, a Quality-of-service (QoS) mapping strategy smartly loads the scalable video to the underlying STBCs, and guarantees that more important video packets obtain higher transmission priorities and stronger protections. Simulation results demonstrate the effectiveness of our proposed scheme.