A power amplifier is provided. The power amplifier includes a loading circuit, a first stage amplifying circuit, an analog pre-distorter, a loading circuit and a second stage amplifying circuit. The first stage amplifying circuit is coupled to the loading circuit to receive a first signal and output a second signal accordingly. The analog pre-distorter is coupled to the first stage amplifying circuit to detect the envelope of the second signal and generates a third signal according to the envelope. The second stage amplifying circuit is coupled to the first stage amplifying circuit to receive the second signal. The loading circuit is biased on the third signal. The gain of the first stage amplifying circuit is related to the third signal.