This paper describes some of the most important design considerations such as: transistor and substrate selection for high power,matching network design and stabilization of microwave power amplifiers with discrete die transistor in X-band frequency. According to these considerations, a 30- watt power amplifier with a GaN-HEMT transistor over 8.8 to 9.8GHz has been designed and fabricated. The power gain,power added efficiency and 1dB compression point of the fabricated circuit are 9.2dB, 53 percent and 29.1watt respectively. Due to the self-heating issue of high power transistors, electro thermal operation of transistor should be well-thought-out, hence the transistor attached on a high thermal conductivity carrier and biased under pulsed condition. The measurement results show good agreement with simulations

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