A heat sink assembly comprises a heat sink (20) and a locking device assembly (30) for mounting the heat sink to an electronic component (44) of a printed circuit board (40). A channel (25) is defined in the heat sink. The locking device assembly comprises a pressing member (31) and a pair of wire clips (33) pivotably engaging with opposite ends of the pressing member. The pressing member spans across the channel in a manner such that the pressing member resiliently abuts against the heat sink. The wire clips are located at opposite lateral sides of the heat sink and engage with clasps (42) on the printed circuit board. Use of the locking device assembly can prevent the heat sink from being held at a slant relative to the electronic component.