A heat sink assembly includes a fan (10), a heat sink (20) and a mounting device (50). The heat sink includes a base (22) and plural of fins (24) extending upwardly from the base. Plural of claws (26) are formed in a top end of one fin which extends from a side of the base. A locking hole (30) is defined in another fin which extends from an opposite side of the base. A vent (52) is defined in the center of the mounting device, and four fixing holes (54) are defined in four comers of the mounting device for engaging with four screws (14) which extend through the fan to the mounting device. Plural of pivots (56) are provided in a first side of the mounting device for pivotably received in the claws of the heat sink thereby pivotably attaching the mounting device to the heat sink. A hook (62) is provided in an opposite side of the mounting device for engaging with the locking hole of the heat sink thereby fixing the mounting device to the heat sink.