A heat dissipation device includes a heat sink (10), a fan holder (20) located on the heat sink and a fan (30) mounted on the fan holder. The heat sink includes a plurality of fins (14) extending therefrom and defines two slots (16) in two opposite outmost fins thereof. The fan holder includes two brackets (200) each having a positioning rib (212) engaged in a corresponding slot of the heat sink and resisting the outmost fin to thereby mount the fan holder to the heat sink.