A 910MHz high efficiency RF transceiver for Wireless Body Area Network in medical application is presented in this paper. High energy efficiency transmitter and receiver architectures are proposed. In wireless body sensor network, the transmitter must have higher efficiency compared with the receiver because a large amount of data is sent from sensor node to receiver of the base station and smaller amount of data is received.  Using colpitts oscillator in the proposed transceiver with injection-lock technique, the efficiency of transmitter and receiver has been improved. The designed transceiver consumes 1 mW at -6 dBm output with 5 Mbps data rate. Energy consumption of transmitter and receiver are 864 pJ/(bit×mW)  and 114 pJ/bit, respectively. The proposed transmitter was designed in 0.18 µm RFCMOS technology.