An auto-balancing apparatus for optical disk drives mainly includes a housing for holding a balance weight and a plurality of clamp mechanisms movable radially through the perimeter of the housing. The clamp mechanisms are engaged with a spindle of a spindle motor or the housing through elastic elements. When the spindle motor is not rotating, the balance weight is anchored at a desired position by the clamp mechanisms because of the elastic force of the elastic elements. When operating at high rotation speeds, the engagement force of the elastic elements exerted on the clamp mechanisms is smaller than the centrifugal force, and the balance weight may be moved freely to automatically reach a balanced condition. When operating at low rotation speeds, the server system drives the spindle motor to rotate at speeds faster than the critical rotation speed to reach a balanced condition, then slows down the rotation speed to allow the engagement force exerted on the clamp mechanisms to be greater than the centrifugal force and anchor the balance weight. Thus the auto-balancing apparatus can achieve a balance effect at low rotation speeds.