An air-cooled Gallium Nitride (GaN)-based solid state power amplifier (SSPA) unit consists of two power amplifier modules, processor and filter boards, mechanical switches, fins and two fans, was designed in compliance with the military standards. Total heat load of SSPA-unit calculated as 196 W including solar radiation load. Although the total power dissipation is in the moderate range, employed GaN-based chip has a transistor level high heat fluxes of about 7300 W/cm(2). Thermal design comprises forming the preliminary mechanical structure of fans, fins, and mechanical interfaces in accordance with the design constraints. Module housing and thermal interface materials were determined by comparing numerical model and measurement results for optimum electrical and thermal performance. Results showed that copper and high-thermal-conductivity epoxy solutions have better thermal performance with more than an order of magnitude mean-time-between-failure improvement over the aluminum and indium foil for module housing and thermal interface material, respectively.