A computer system includes a chassis, a motherboard secured to the chassis, and a heat sink. A chip is mounted on the motherboard. A securing component secured to the motherboard adjacent the chip. The securing component defines an opening. Two positioning posts are located on the motherboard and adjoin two opposite edges of the opening. Each positioning post defines a fastener hole. A heat sink includes a main body and a fin portion located on the fin portion. The main body defines two through holes. The main body defines two recesses to expose the through holes. The heat sink is secured to the securing component, by two fasteners engaged in the through holes and the fastener holes.