A heat sink assembly includes a heat sink and a clip. The heat sink includes a base and a plurality of fins extending upwardly from a top surface of the base. The clip includes a pressing body, two bending portions slantwise to the pressing body and extending from two opposite ends of the pressing body, and two locking arms extending from two ends of the bending portions. The pressing body of the clip abuts against the top surface of the base. The bending portions are extended downwardly from the pressing body to be located adjacent to two sides of the base to prevent the clip from moving horizontally along a lateral direction of the heat sink.