The operating principle of typical MOSFETs AC/DC rectifier is introduced in this paper. In order to maximize the operating range of RFID transponder, low-power design techniques are needed. Therefore, the key design parameters optimization of passive rectifier is discussed. Besides, the design of a low-power rectifier for passive UHF RFID transponder, which is compatible with standard CMOS process and can be applied to the environment in which the distance from interrogator changes greatly, is also presented in this paper. Measurement results showed that if a 510 kilo-ohm resistance is added at the rectifier output and 4 W EIRP interrogator transmit power, the rectifier can output 1.6 V to 2 V DC voltage, the minimum RF input power is about 230 muW corresponding to a reading distance of 3.45 m.