A locking device for mounting a heat sink to a CPU mounted on a printed circuit board, includes a back plate mounted below the circuit board and having a plurality of fasteners extending upwardly beyond the circuit board, and a plurality of retainers. Each retainer includes an operating member resting on a heat sink, a locking member extending through the operating member, and a pushing member extending through the locking member. The locking member includes a locking structure extending through the heat sink to engage with one of the fasteners. The pushing member includes a taper bottom portion. The pushing member is downward movable to cause the taper bottom portion to disengage the locking structure from the one of the fasteners.