A computer system includes a chassis (10), a securing panel (30), a motherboard (20), and a heat sink (40). The chassis includes a plate (12) having at least one positioning post (14) extending upwardly therefrom. The securing panel defines a plurality of screw holes (38) and at least one positioning hole (34) for the at least one positioning post extending therethrough. The motherboard is placed on the securing panel and secured with the chassis. The motherboard includes a plurality of through holes (22) corresponding to the plurality of screw holes of the securing panel and an electronic component (50) located among the through holes. The heat sink with a plurality of fastening bolts (42) placed on the electronic component. The fastening bolts of the heat sink are screwed into the through holes of the motherboard and the screw holes of the securing panel to fasten the securing panel and the heat sink at opposite sides of the motherboard.