This paper presents a novel ultra-low input power self-bias CMOS rectifier for ultra-high frequency (UHF) RF energy harvester, which achieves a high power conversion efficiency (PCE) at an ultra-low input power. A six stages rectifier is designed using this proposed topology in a 130nm CMOS process technology. Operating at 915MHz and driving a 1MΩ load resistor, the post-layout simulation PCE of this work is 27.6% at -30dBm input power. A sensitivity of -30dBm is measured with 0.8V output voltage across a capacitive load.