Abstract: A design methodology for multi-octave power amplifiers (PAs) is presented in this paper using sequential nested load-pull technique. Unlike conventional load-pull technique, the proposed method considering the correlation between the fundamental impedance and the second harmonic impedance can provide more accurate design area. And it can not only investigate the in-band harmonics influence on the multi-octave performance of the PA, but also can improve the design area making the broadband matching network design more convenient. To demonstrate the capability of this technique, an ultra-wideband PA prototype is designed and fabricated. Experimental results show that the drain efficiency is 61. 8%-73. 6% and the output power is 40. 2-42. 6 dBm across 0. 3-3. 2 GHz (relative bandwidth 165. 7%). The proposed PA obtains 7. 4 cm×3. 0 cm area, which is reduced comparing with other PAs in similar performances.