This paper reports a high efficiency class-F power amplifier based on a gallium nitride high electron mobility transistor (GaN-HEMT), which is designed at the L band of 1640 MHz. The design is based on source and load pull measurements. During the design process, the parasitics of the package of the device are also taken into account in order to achieve the optimal class-F load condition at the intrinsic drain of the transistor. The fabricated class-F power amplifier achieved a maximum drain efficiency (DE) of 77.8% and a output power of 39.6 W on a bandwidth of 280 MHz. Simulation and measurement results have shown good agreement