A fastener comprises a fastener post, a clasping element and an elastic element. The fastener post comprises a head, a post body and a threaded portion. The post body has a first extension section connecting with the head, a second extension section connecting with the threaded portion and a recess section connected between the first and second extension sections. The clasping element comprises an annular clasping body, a first clasping portion axially extending from the clasping body and a second clasping portion extending outwardly and then downwardly from the clasping body. The clasping body encloses the post body, the first clasping portion engages with the recess section of the post body, and the second clasping portion engages with the heat sink. The elastic element is set around the post body and located between the head and the clasping body.