A heat dissipation device comprises a heat sink, a fan, and a fan holder. The heat sink includes a base and a plurality of fins arranged on the base. The fan includes a frame having an engaging flange at each corner thereof. The fan holder configured for attaching the fan to the heat sink includes a main body having two opposite sidewalls extending upwardly from two opposite sides of the main body, a peg extending through the sidewall and movable in a direction perpendicular to the sidewall, and a handle capable of operating the peg. At least one of the engaging flanges of the fan is depressed by the peg towards the main body, thereby securing the fan on the fan holder.