A fastener includes a fastener post, a sleeve, a nut and an elastic element. The fastener post includes a shaft, a tapered portion and a threaded portion. The sleeve includes a smooth portion and a clamp portion having a number of resilient strips. The nut is threaded onto the threaded portion. The elastic element is disposed around the sleeve and compressed between the nut and an ear of a heat sink to generate a resilient force. The resilient force moves the tapered portion of the fastener post into the sleeve such that the strips of the clamp portion of the sleeve are tilted outwardly to tightly abut against a bottom of a printed circuit board on which the heat sink is mounted.