A heat sink assembly includes a heat sink, two first fixing structures, and two second fixing structures. The heat sink includes a base. The base includes two opposite sidewalls each forming a fixing portion. When the heat sink is mounted to a first motherboard, the first fixing structures are respectively and detachably connected to the fixing portions of the heat sink, and fix the heat sink to the first motherboard. When the heat sink is mounted to a second motherboard, the second fixing structures are detachably connected to fixing portions of the heat sink, and fix the heat sink to the second motherboard.