This paper presents a low power rectifier, which is compatible with standard CMOS process, and a novel OOK(On-Off Keying) demodulator technique for passive UHF RFID transponders. This technique has some outstanding merits, such as, low power, being easy to be integrated and can be applied to the environment, whose distance from the reader changes greatly. We simulate the circuits with BSIM3V3.2 model and Cadence Spectra. Those circuits have been realized on 0.18μm standard CMOS process. The measurement results shown that if added a 510KΩ resistance at the power output and 4W EIRP(equivalent radiated power), the rectifier can output 1.6V to 2V, corresponds to a reading distance of 3.7m.