—This paper studies the energy efficiency (EE) ofthe two-way wireless communication system operating in thefull-duplex (FD) and half-duplex (HD) modes, respectively. Particularly, both the residual self-interference (RSI) and powerconsumption for the self-interference cancellation (SIC) in the FDmode are modeled as linear functions over the transmit power.The EE maximization problems for the FD, time division duplex(TDD), and frequency division duplex (FDD) modes with the sumand individual spectral efficiency (SE) constraints are studied,respectively. With the sum SE constraint, closed-form expressionsfor the maximum EE of these three modes are derived, andthe maximum EE among them are compared. Based on thecomparison results, a duplex selection scheme to achieve thehighest EE among the three duplex modes is proposed. With theindividual SE constraint, the optimal and suboptimal resourceallocation are obtained by utilizing fractional programming forthe three duplex modes. Somehow surprisingly, numerical resultsreveal that the FD mode achieves the best EE performance whenthe target sum SE or the distance between the two transceiversis relatively large.