A heat sink fastener includes a main body, a piercing body and an operating member. The main body includes a pressing part adapted for pressing a heat sink toward a heat-generating component, an engaging part and a latching leg, the engaging part and the latching leg being disposed at opposite ends of the pressing part. The piercing body includes a piercing part piercing through the engaging part and movable up and down in the engaging part, and a latching part below the piercing part and facing the latching leg. The operating member is connected to the piercing part and linearly movable along a direction perpendicular to the engaging part to put the pressing part in tension by exerting a force on the engaging part or to relax the tension by relieving the force exerted on the engaging part.