A radio frequency (RF) amplifier is disclosed. The RF power amplifier includes a bias circuit, an output-stage circuit and a RF compensation circuit. When a first system voltage is larger than a first voltage threshold value, the bias circuit generates a first current rising slightly. When first system voltage is larger than second voltage threshold value, the RF compensation circuit receives a second circuit rising slightly transmitted from the bias circuit. When the first system voltage is in an operation voltage range, the first current is larger than the second circuit so as to a quiescent operating current of the RF power amplifier is independent of change of the first system voltage. When the first system voltage is larger than a third voltage threshold value, the first current is equal to the second current so as to have the bias current being a zero current to protect the RF power amplifier from over-voltage.