A clamping apparatus includes a base, a support mechanism mounted on the base, and a plurality of retaining mechanisms mounted on the base. A wheel rim is mounted on the support mechanism of the clamping apparatus to be worked by a wheel rim working machine. Thus, each of the retaining mechanisms of the clamping apparatus presses the inner wall of the wheel rim to position the wheel rim in place when the wheel rim is worked by the wheel rim working machine, so that the wheel rim is positioned by the clamping apparatus, and will not slip or detach from the clamping apparatus due to the high speed operation of the wheel rim working machine.