A low noise amplifier is disclosed. The low noise amplifier comprises a current mirror circuit, a bias circuit, a cascode amplifying circuit and a power gain compensating circuit. The current mirror circuit is used for providing a first current and third current. The bias circuit is used for receiving a first current and third current and outputting a first bias voltage and a second bias voltage according to the first current and third current. The cascode amplifying circuit respectively receives the first bias voltage and the second bias voltage, and accordingly to work at an operation bias point. The power gain compensating circuit is used for receiving a RF output signal and accordingly outputs a gain compensating signal to the current mirror circuit so as to dynamically adjust current value of the first current and third current and further to compensates power gain of the low noise amplifier in order to increase 1 dB gain compression point (P1 dB).