A 4 W K-band AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor(PHEMT) monolithic microwave integrated circuit(MMIC) high power amplifier(PA) is reported.This amplifier is designed to fully match for a 50Ωinput and output impedance based on the 0.15μm power PHEMT process.Under the condition of 5.6 V and 2.6 A DC bias,the amplifier has achieved a 22 dB small-signal gain,better than a 13 dB input return loss, and 36 dBm saturation power with 25%PAE from 19 to 22 GHz. A 4 W K-band AlGaAs / InGaAs / GaAs pseudomorphic high electron mobility transistor (PHEMT) monolithic microwave integrated circuit (MMIC) high power amplifier (PA) was reported. This amplifier is designed to be fully match for a 50Ω input and output impedance based on the 0.15 μm power PHEMT process. Having the condition of 5.6 V and 2.6 A DC bias, the amplifier has achieved a 22 dB small-signal gain, better than a 13 dB input return loss, and 36 dBm saturation power with 25% PAE from 19 to 22 GHz.