A cooling device includes a heat sink (10), a fan (30), and a fan duct (50) covering the heat sink and fan therein. The heat sink includes a base (12) defining at least a through hole (18) at each of two opposite sides thereof. The fan duct comprises a top wall (52) and two side walls (54). A mounting portion (60) extends outwardly from a bottom of each side wall and has at least a tube (64) defining a mounting hole (640) in an outer wall thereof. A plurality of pins (70) extends through the through holes into the tubes. The pins have retaining rings (74) abutting against a bottom surface (120) of the base of the heat sink, and hooks (76) locked in the mounting holes.