This paper proposes a power-efficient mode selection and power allocation scheme in device-to-device (D2D) communication system as an underlay coexistence with cellular networks. The proposed scheme is performed based on the exhaustive search of all possible mode combinations of the devices which consist of the mode indices for all devices in the system. Specifically, the proposed scheme consists of two steps. First, we calculate the optimal power with respect to the maximum power-efficiency for all possible modes of each device. Since the power-efficiency is not a concave function for the transmission power, we obtain the suboptimal solution by using the concavity of the lower and upper bound for the power-efficiency. The powerefficiencies for all possible modes of each device are obtained by the suboptimal power allocation in the first step. In the second step, we select a mode sequence which has the maximal power-efficiency among all possible mode combinations of the devices based on the obtained power-efficiencies in the first step. Then we can jointly obtain the suboptimal transmission power and the mode maximizing the power-efficiency. The proposed suboptimal scheme for the power allocation and mode selection performs close to the upper bound with respect to the power-efficiency. The simulation results also show that the proposed scheme outperforms the conventional schemes with respect to the power-efficiency and system capacity.