Abstract: The invention relates to a locking device mounted directly on a circuit breaker and configured for mounting different locks. The locking device includes a yoke member, a slider member, a handle block and a lock. When the locking device is locked, the handle block abuts the operating handle in an engaged position corresponding to the operating handle in the ON or OFF position. Thus, the locking device locks the circuit breaker in the OFF position or the ON position. The locking device may further comprise a base member disposed between the yoke member and the housing. When it is desired to lock the circuit breaker in the on position, the base member is adapted to cover the push-to-trip button on the housing.