This letter proposes a dual-band high-efficiency rectifier using a GaN transistor. Positive polarity and negative polarity modes are set at f1 , and f2 , respectively. Then, the rectifier operation at f1 and f2 is realized upon adjusting only the phase of the corresponding feedback loop. This releases the limitation that the original power amplifier (PA) must be dual-band operation. In addition, class-F harmonic control theory is used to enhance efficiency. Moreover, the proposed technique has no frequency dependence and can be used to achieve arbitrary frequency ratios. To verify the effectiveness of the proposed method, a high-efficiency rectifier operating at 1.9 and 2.4 GHz is designed and fabricated using a CGH40010F GaN HEMT. Measurements illustrate that the implemented rectifier can realize an efficiency of over 75% at 1.9 GHz and 2.4 GHz when an input power of 40 dBm is applied and the dc load is 70 Ω . Also, the size of circuit is only 4.7 cm x 3.4 cm.