A fastener for a heat sink of the present invention includes a length-variable operating member (10), a piston member (20), an embracing member (30), a resilient member (40) and a post (58) extending from a printed circuit board (50). The piston member is movable in the operating member when the operating member varies between a shortest length and a longest length. The resilient member covers around the piston member. The embracing member is controllable cooperatively by the piston member and the resilient member to grasp the post or release the post.