In this paper, an X-band 50 W internally matched power amplifier is designed and fabricated using an 80×150 μm GaN HEMT that is developed by the 0.25 μm GaN HEMT process of ETRI. The optimum source and load impedances are experimentally extracted from the loadpull measurement using impedance-transform-prematching circuits, and the transistor performance is predicted. The power performance of the internally matched power amplifier, whose matching circuits are fabricated on a substrate with εr of 10.2, is measured under the pulsed mode of 100 μs pulse period and 10 % duty cycle, and the best output power of 47.46 dBm(55.5 W) and the power-added efficiency of 46.6 % are obtained at 9.2 GHz. The output power of 47～47.46 dBm(50～55.7 W) is measured in 9.0～9.5 GHz, and the power-added efficiency is measured to be greater than 43 % in 9.0～9.3 GHz and above 36 % in 9.4～9.5 GHz.

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