An operational amplifier, including an input stage circuit, an output stage circuit and a constant-gm circuit is provided. The input stage circuit provides a driving voltage according to an input voltage. Input terminals of the output stage circuit receive the driving voltage, and an output terminal of the output stage circuit provides an output voltage according to the driving voltage. The constant-gm circuit includes a constant-gm switch circuit, a current mirror circuit and a current mirror switch circuit. The constant-gm switch circuit controls the operation of the constant-gm circuit. The constant-gm switch circuit allows the current mirror circuit to operate during a transient time when the driving voltage is transited according to the driving voltage, so as to provide a compensated current for the input stage circuit. When the driving voltage is during a non-transient time, the current mirror switch circuit turns off the current mirror circuit.