A heat dissipation device includes a heat sink, a fan and a fan holder attaching the fan to the heat sink. The heat sink includes a base and a plurality of fins. The fan includes a frame and four engaging flanges at a periphery thereof. The fan holder includes a supporting board supporting the fan thereon, a plurality of spaced retaining plates extending downwardly from a periphery edge of the supporting board and clasping the fins, two locking sheets extending upwardly from two diagonal corners of the supporting board and two locking poles extending upwardly from another two diagonal corners of the supporting board. The locking sheets abut against lateral sides of two engaging flanges of the fan. The locking poles are engagingly received in another two engaging flanges of the fan.