An X-band high-power amplifier (HPA) based on gallium nitride (GaN) high electron mobility transistors (HEMTs) has been developed for synthetic aperture radar (SAR) applications. A hybrid power combining technique, including microstrip circuits and waveguides, is used to design the HPA. For reducing the size, four 50 W GaN HEMTs cascaded with one 1-to-4 power divider and one 4-to-1 power combiner form a 4-way power combined PCB circuits. For combing the high power and driving an antenna, two PCB circuits are combined by magic-T waveguides. The transmission efficiency of the power combining is approximately 80%. In the 10% duty cycle (pulse width 100 us), the output power of the HPA is over 200 W across the band of 9.5–9.8 GHz. The maximum output power is 230 W at 9.5 GHz, and the power gain is 8.3 dB at 46.1°C.