In the case of transmitting stored video streaming over packet-switched networks, unavoidable frame losses may result in error propagation of reconstructed video and thus induce severe quality degradation. In order to minimize the transmission distortion and utilize the limited channel resources efficiently, unequal error protection is usually adopted to exploit the unequal importance of different frames in one group-of-pictures (GOP). In this paper, we develop an estimate model for GOP-level transmission distortion so that the transmitter is able to find the unequal importance of different frames in one GOP. The simulation results demonstrate that the proposed model is accurate and robust.