A locking device for mounting a heat dissipating device to a CPU mounted on a printed circuit board, includes a back plate mounted below the circuit board and a retainer. The back plate includes a plurality of posts extending upwardly through the circuit board and engaging with screws thereat. The retainer includes a mounting section attached to the heat dissipating device and a plurality of spring locking sections extending radially from the mounting section. The locking sections are downwardly pressable to engage with the respective posts and screws whereby the retainer mounts the heat dissipating device to the electronic component.