A power amplifier integrated circuit, which generates an RF output signal by amplifying an RF input signal, includes a thermal-sensing circuit, a feedback circuit, a logic judging circuit, an adjusting circuit, and an amplifying circuit. The thermal-sensing circuit generates a thermal sensing signal according to the operational temperature, and the feedback circuit generates a power compensation circuit according to power variations in the RF output signal. The logic judging circuit outputs a compensation signal according to the thermal sensing signal and the power compensation signal. The adjusting circuit adjusts the level of the RF input signal according to the compensation signal, thereby generating a corresponding 1st stage RF signal. The amplifying circuit can amplify the 1st stage RF signal, thereby generating the corresponding RF output signal.