This brief presents a fully integrated automatic impedance-matching RF energy harvesting system that automatically compensates for variations at the antenna-rectifier interface. The control loop is fully integrated on-chip with a low-voltage, low-power design. The design is fabricated in 0.18-µm CMOS technology, and the active chip area is 0.064mm². The sensitivity optimized by impedance tuning can be improved to more than -30.5dBm for 0.7V output at varied initial mismatches, leading to a maximum increase in sensitivity of 5dB. The control circuit’s power consumption is only 2.4nW, and the minimum supply voltage is 416mV.