This paper proposes an adaptive power rectifier at 915 MHz operating in a wide range of low input power. A field effect transistor works as an adaptive switch for changing the working modes of the rectifier according to the input power level. As a result, the rectifier can achieve high MW-DC conversion efficiency over the entire range. A double-stub matching network matches the input impedance of the diode to 50 . A shunted capacitor works as a DC-pass filter to smooth the output DC power. Simulation results show that efficiency is above 40% within an input power range from −16 to 4 dBm, and the maximum efficiency is 66.2%. The measured center frequency shifts a litter and the maximum efficiency is 47.5%. The rectifier is fabricated using a simple planar printed circuit board (PCB) technique, and is easy to be integrated. It can be used to collect ambient electromagnetic energy.