A heat dissipating apparatus for dissipating heat generated by a VGA card includes a heat sink and a retention assembly. The heat sink includes a base located on the add-on card, a plurality of fins, a fan for blowing air toward the fins and a cover for covering the fins and the fan. The retention assembly includes four pegs each having a first end locked in the base of the heat sink and a second end, a back plate secured beneath the add-on card and two wire clips. The second ends of the pegs extend through the base, the add-on card and the back plate in turn and are held in position by the wire clips beneath the back plate, whereby the heat sink and the back plate are secured to opposite sides of the add-on card.