An operational amplifier circuit is provided. The operational amplifier circuit includes a differential input stage circuit and a loading stage circuit. The differential input stage circuit includes an input circuit, a voltage maintaining circuit, and a current source. The input circuit includes a first input transistor and a second input transistor, for receiving a first and a second input signals, respectively. The voltage maintaining circuit includes a first branch circuit and a second branch circuit. The first branch circuit is coupled to the first input transistor for receiving the first input signal, and the second branch circuit is coupled to the second input transistor for receiving the second input signal. The current source is coupled to the first input transistor and the second input transistor. The loading stage circuit is coupled to the voltage maintaining circuit.