An operational amplifier capable of enhancing slew rate is disclosed. The operational amplifier includes a first current generator for generating a first bias current, a second current generator for generating a second bias current, an amplification stage, coupled to the first current generator, for generating a amplification signal according to an input signal, an output stage, coupled to the second current generator and the amplification stage, for generating an output signal according to the amplification signal, and a bias current allocation unit, coupled to the first current generator, the second current generator, the amplification stage and the output stage, for reallocating current intensities of the first bias current and the second bias current according to a control signal.