Two types of CMOS rectifier circuits for UHF RFIDs have been proposed. Large power conversion efficiency (PCE) has been achieved by Vth cancellation techniques. In the Self-Vth-Cancellation (SVC) CMOS rectifier, the threshold voltage of MOSFETs is cancelled by gate bias voltage generated from the output voltage of the rectifier itself, resulting in excellent PCE. The differential-drive CMOS rectifier has a cross-coupled bridge configuration and is driven by a differential RF input. A differential-drive scheme realizes an active gate bias mechanism and simultaneously enables both low ON-resistance and small reverse leakage of diode-connected MOS transistors, resulting in large PCE, especially under small RF input power conditions. Test circuits for both types of rectifiers were fabricated and the measured performances were compared with those of conventional rectifiers.