The proposed heat sink assembly mounted on a heat source includes a heat sink including a base with a top surface and a plurality of fins, and a retaining device including a frame with a top, a hole with a size larger than that of the base, an elastic rod connected with the frame, extending into the hole and having and end portion protruding towards the hole and pushing the heat sink and a plurality of retaining plates extending downwards from one of the first two and the second two opposite sides. Each of the retaining plates has a barb mounting the heat sink on the heat source and making the top surface raise through the hole to a same height of the top to facilitate the heat dissipation.