A high sensitivity Class-3 ultra high-frequency (UHF) radio-frequency identification (RFID) system based on harmonic backscattering to achieve a long read-range is presented. Harmonic backscattering is a promising solution to solve the carrier leakage issue of UHF RFID reader to improve the sensitivity. In the proposed RFID system based on harmonic backscattering, the RFID reader transmits data with the fundamental frequency carrier and receives data with its second harmonic carrier, and the RFID tag employs a rectifier to generate both DC power and the second harmonic carrier for backscattering without impedance mismatch. The first harmonic Class-3 UHF RFID system based on National Instruments software defined radio platform is proposed and implemented. The proposed semi-passive harmonic RFID tag has dBm sensitivity. Test results show that the harmonic RFID system has a dBm sensitivity with the harmonic RFID tag whose signal-to-blocker ratio is dB at 20 m communication distance.