The invention is directed to the selectable mode buffer circuit including a plurality of pads, a mode selecting circuit, and a control circuit. The mode selecting circuit has a plurality of switches, coupled to the pads, and performs a charge pumping operation or an interfacing operating by changing on or off status of at least one of the switches according to a mode selecting signal. The control circuit receives the mode selecting signal, and generates a plurality of input signals for controlling the switches according to the mode selecting signal.