Abstract A CMOS cascode class E power amplifier has been designed at 5.2 GHz. Its RF performances such as output and power-added efficiency have been examined in ADS simulation. The layout parasitic is accounted for in the post-layout simulation. Time-dependent drain–source voltage waveforms indicate that the drain of cascode transistor is subject to much higher voltage stress than that of main transistor. Analytical equation of output power including impact of gate-oxide breakdown is developed and compared with RF simulation results. Good agreement between the model predictions and ADS simulation is obtained. The gate–drain breakdown of the cascode transistor decreases the output power and power-added efficiency of the power amplifier significantly when the breakdown resistance is below 1 kΩ.