A heat sink assembly includes a first heat sink, a second heat sink, and a wire clip connecting the first and second heat sinks together. Each of the first and second heat sinks includes a base and a plurality of connecting members extending upwardly from the base. The wire clip includes a pivot portion and a pair of pressing portions extending from opposite ends of the pivot portion towards each other. The connecting members of the first and second heat sinks cooperatively clasp the pivot portion of the wire clip. The pressing members of the wire clip resiliently abut against the connecting members of the first and second heat sinks and push the connecting members of the first and second heat sink outwardly away from each other. The bases of the first and second heat sinks clamp an add-on card therebetween.