A heat dissipating device used for dissipating heat from a heat generating device (100) in a computer. The heat dissipating device includes a fan bracket (20), a fan (27) received in the fan bracket, and an air duct (50). The fan bracket defines a fan opening (25), and has a supporting member (233). The supporting member has a flat board (2333). A pair of retaining portions (2335) is formed on opposite edges of the flat board respectively. The flat board defines a retaining hole (2337) therein. The air duct defines an airflow passageway and has a top wall (51) and two opposite sidewalls extending from the top wall. A securing member (553) extends from one of the sidewalls is retained between the two retaining portions. A post (5535) protruding from the securing member is engaged in the retaining hole for cooperatively retaining the air duct to the fan bracket.