An uninterruptible power supply apparatus includes a power supply unit converting AC power to generate a DC voltage to be supplied to a loading device. The uninterruptible power supply apparatus further includes a first battery unit connected to the power supply unit to receive a DC current therefrom, and stores first DC power generated from the DC voltage. The first battery unit generates a DC voltage to be supplied to the loading device from the first DC power. The uninterruptible power supply apparatus includes a second battery unit connected in parallel to the power supply unit and stores second DC power, and generates, from the second DC power stored in the second battery unit, a DC voltage to be supplied to the loading device. The uninterruptible power supply apparatus further includes a controller for controlling operation of the power supply unit, the first battery unit and the second battery unit.