This paper reports a CMOS power amplifier (PA) designed considering the performance degradation of MOSFETs caused by both temperature variations and the hot-carrier effect. A small-signal MOSFET model that reproduces the transient degradation caused by the hot-carrier effect is proposed. A 79 GHz CMOS PA was designed utilizing the proposed small-signal model. Simulation results agreed well with the measurement results of the PA fabricated with a 40 nm CMOS technology. It achieved the small-signal gain variations of below 0.7 dB and the OP1dB degradation of less than 0.8 dB in the temperature range of 0°C to 100°C.