Abstract: In this paper, a 780 MHz high PAE Class D RF CMOS power amplifier with tunable output power in a standard 65 nm COMS process is presented for IOT applications. A DC-DC buck converter is introduced to modulate the supply voltage of the PA while a tunable output matching network is employed to provide two different equivalent load resistances to adjust the output power. Simulation results indicate that the proposed power amplifier provides output power from -0.2 dBm to 9.1 dBm and the PAE higher than 60% at 1.2 V power supply.