A tournament-shaped magnetically coupled powercombiner architecture for a fully integrated RF CMOS power amplifier is proposed. Various 1 : I transmission line transformers are used to design the power combiner. To demonstrate the new architecture, a 1.81-GHz CMOS power amplifier using the tournament-shaped power combiner was implemented with a 0.18-mu m RF CMOS process. All of the matching components, including the input and output transformer, were fully integrated. The amplifier achieved a drain efficiency of 38 % at the maximum output power of 31.7 dBm.