A semi-passive ultrahigh frequency (UHF) radio frequency identification (RFID) system is presented. The reconfigurable architecture of tag is proposed to be compatible with passive and active operating modes. The read range of RFID system and the lifetime of tag is increases by frequency locked on-chip oscillator topology. The transceiver is implemented in 0.18μm standard CMOS process. The sensitivity of tag is -18dBm for passive mode and -40dBm for active mode with 1.2μW power consumption under the supply voltage of 0.8V. The output power is -10dBm for active transmitting mode and the power consumption is 450μW under the supply voltage of 1V.