The present invention relates to an output stage circuit and an operational amplifier thereof. In the output stage circuit, one of a gate of a transistor is coupled to a gate of a bias transistor and a level shifter in response to a small signal outputted from an amplifying circuit in the operational amplifier. In addition, a gate voltage of the bias transistor is controlled by a voltage generating circuit to control a DC bias of the transistor of the output stage circuit. Therefore, extra frequency compensating components for compensating the transistor of the output stage circuit is no longer necessary, and saving circuit layout area and cost can be achieved by the present invention.