We report, in this paper, a highly efficient CMOS rectifier for low power energy harvesting system. By using full bridge structure and a novel active diode, the rectifier can work at a wide range of input voltage amplitudes of 0.45V up to 1.8V under a standard 0.18µm CMOS process. A comparator that consists of common-gate stage and control stage is designed to control the switch of the active diode. The proposed rectifier can achieve a peak voltage conversion efficiency of over 96% and a power efficiency over 90%. Simulated power consumption of the rectifier is 264.35nW at 500mV, which is much smaller than the best recently published results.