This letter presents a simple method to design dual-band high-efficiency power amplifiers (PAs). The dual-band coupler is exploited to design the output circuits of the PA. By proper selection of impedance conditions of terminal ports, the dual-band coupler can convert the fundamental impedance of transistor to the standard 50-Ω load at two frequencies. At the same time, the second harmonic impedance is also controlled without the need for additional tuning. For validation, a dual-band high-efficiency PA is designed and fabricated by using CGH40010F GaN HEMT. Measurements indicate that the designed PA can deliver saturated output power of 41.6 and 42.1 dBm at 1 and 2.3 GHz, respectively. Also, the obtained drain efficiency exceeds 72% at both frequencies.