A heat sink assembly for an add-on card includes a heat sink and a clip received in the heat sink. The heat sink includes a supporting plate and a first and a second heat absorbing plates extending downwardly from the supporting plate. The first and second heat absorbing plates sandwich first and second heat conductive plates and the add-on card therebetween. The supporting plate is located over and spaced from the add-on card. The clip includes a resisting member, first and second engaging members and first and second pressing members. The resisting member is received in the heat sink and abuts upwardly against the supporting plate of the heat sink. The first and second engaging members engage with the first and second heat absorbing plates, respectively. The pressing members abut downwardly against a clasp clasping the conductive plates and the add-on card together.