A heat dissipation device for an electronic unit includes a heat sink (10), a pair of fan holders (30, 40) and a fan (50) mounted on the heat sink via the fan holders. The heat sink includes a base (11) with a plurality of fins (15) extending upwardly therefrom along a lateral direction thereof. The base forms a pair of lateral walls at two opposite lateral sides thereof. The fins are sandwiched between the lateral walls. A pair of supporting plates (17) extend from the lateral walls for mounting the fan holders on the heat sink. The fan holders cooperate with the lateral walls of the heat sink to encompass a top of the fins to prevent airflow generated by the fan from escaping from the top of the fins.