A C-band high efficiency and high gain two-stage power amplifier based on Al Ga N/Ga N high electron mobility transistor(HEMT) is designed and measured in this paper. The input and output impedances for the optimum power-added efficiency(PAE) are determined at the fundamental and 2nd harmonic frequency( f0 and 2 f0). The harmonic manipulation networks are designed both in the driver stage and the power stage which manipulate the second harmonic to a very low level within the operating frequency band. Then the inter-stage matching network and the output power combining network are calculated to achieve a low insertion loss. So the PAE and the power gain is greatly improved. In an operation frequency range of 5.4 GHz–5.8 GHz in CW mode, the amplifier delivers a maximum output power of 18.62 W, with a PAE of 55.15% and an associated power gain of 28.7 d B, which is an outstanding performance. A C-band high efficiency and high gain two-stage power amplifier based on Al Ga N / Ga N high electron mobility transistor (HEMT) is designed and measured in this paper. The input and output impedances for the optimum power-added efficiency ( PAE) are determined at the fundamental and 2nd harmonic frequency (f0 and 2 f0). The harmonic manipulation networks are designed both in the driver stage and the power stage which manipulate the second harmonic to a very low level within the operating frequency band. Then The inter-stage matching network and the output power combining network are calculated to achieve a low insertion loss. In the operation frequency range of 5.4 GHz-5.8 GHz in CW mode, the amplifier delivers a maximum output power of 18.62 W, ​​with a PAE of 55.15% and an associated power gain of 28.7 d B, which is an outstanding performance.