A heat dissipation device includes a heat sink (20) having a plurality of fins (26), a fan duct (50), a fan (70) and a mounting bracket (60) for mounting the fan duct and the fan to the heat sink. The fan duct is mounted to a front side of the heat sink, and includes an inlet, an enlarged outlet covering the front side of the heat sink and at least two channels (56). The fan duct is capable of expanding an airflow generated by the fan by the enlarged outlet and dividing the airflow by the at least two channels into at least two sub-airflows. Thus, the fan can blow the airflow through all of the fins to thereby promote a heat dissipating efficiency of the heat dissipation device.