A novel hybrid model selection method between Half-Duplex (HD) and Full-Duplex (FD), is proposed in this paper. The aim of X-Duplex is to maximize the instantaneous system capacity. The system can be configured as either HD or FD mode, by adaptively configuring the antenna as either transmit or receive antenna. HD halves the symbol rate for avoiding the self interference while FD achieves the full symbol rate but may be suppressed by the self interference. X-Duplex chooses the optimal mode, which maximizes the capacity and avoids the serious self interference conditions. By deriving the distributions, we investigate the system capacity and show superiority to the pure duplex modes HD and FD. Simulation results validate our analysis.