In this study, we propose a design methodology for a switching-mode RF CMOS power amplifier with an output transformer. For a given supply voltage, output power, and target efficiency, the initial values of the transistor size, output inductance, and capacitance can be sequentially determined during the design of the power amplifier. The breakdown voltage of the power transistor is considered in the design methodology. To prove the feasibility of the proposed design methodology, we provide the design example of a 2.4-GHz switching-mode CMOS power amplifier with 180-nm RF CMOS technology. From the measured results, the feasibility of the proposed design methodology is proved.