A heat sink fastener includes a main body, a piercing body and an operating member. The main body has a pressing part for pressing a heat sink toward a heat-generating component. An engaging part and a latching leg are disposed at opposite ends of the main body. The piercing body includes a piercing part piercing through the engaging part, and a latching part below the piercing part. The operating member includes a cam pivotally connected to the piercing part and rotatable between locked and unlocked positions. At the locked position, a protrusion formed on the cam abuts a lateral edge of the piercing part to prevent the operating member from spontaneously rotating back to the unlocked position. A bent flange is formed at a periphery of the cam and engages with the engaging part.