A heat dissipation device includes two heat sinks (30) mounted on two electronic components (16) which is mounted within a server (10), two fans (40) mounted on a mounting plate (14) of the server facing the heat sinks for blowing cooling air to the heat sinks, and a fan duct (20). The fan duct includes a top plate (24), a pair of side plates (22) extending from opposite sides of the top plate, a pair of partition walls (26) extending from a middle portion of the top plate, and a stop wall (264) connected between ends of the partition walls. The fan duct is hermetically connected to the mounting plate and the bottom plate thereby cooperatively forming a sealed passage therebetween. The passage includes an inlet (28) adjacent the fans and two separate outlets (29) between the side plates and the partition walls respectively for accommodating the heat sinks therein.