A methodology for designing multioctave high-efficiency power amplifiers (PA) is presented in this letter. The sequential load-pull technique (SLPT) considering the second and third harmonic characteristics is proposed and combined with the multiple frequency matching theory (MFMT) to achieve outstanding performance in terms of operating bandwidth as well as efficiency. To verify the proposed theory, a 10 W GaN HEMT device is used. The proposed PA achieves a fractional bandwidth of 152% from 0.6 to 4.5GHz. In such a wide frequency band, the drain efficiency (DE) is measured as 50%-68% with an output power of greater than 38dBm and a large signal gain greater than 8dB.