A pump is capable of inflating and deflating an inflatable object. The pump includes a cylinder, a piston and a nozzle. The cylinder defines an outlet through which air can only flow into the cylinder and an inlet through which air can only flow from the cylinder. The piston reciprocates in the cylinder. The nozzle defines a first hole for communication with the inflatable object and a second hole for communication with the exterior. The nozzle is rotationally mounted on the cylinder between an inflating mode and a deflating mode. In the inflating mode, the first hole is communicated with the outlet, and the second hole with the inlet. In the deflating mode, the first hole is communicated with the inlet, and the second hole with the outlet.