Disclosed herein is a battery pack having locking members formed at an upper cover and locking grooves formed at a case body such that the locking members can be engaged in the corresponding locking grooves. The assembly of the battery pack is accomplished by a mechanical coupling method, not by an ultrasonic welding method, and the battery pack is manufactured with a high-strength material. Consequently, the battery pack can be manufactured while the thickness of the battery pack is further decreased.