In this paper, we investigate a scenario of D2D-relay communications where one D2D user may help cell-edge user to exchange information for improving its quality of service (QoS). We formulate a resource allocation problem, which aims at maximizing the data rate of the cell-edge user. In particular, we propose an iterative power allocation algorithm and derive the optimal closed-form power allocation expressions by Lagrangian dual method. Simulation results verify the theoretical solution and show that our D2D-relay scheme achieves higher spectrum efficiency than the traditional cellular-relay communication scheme.