A heat sink including an inner piece and an outer piece is provided. The inner piece has a side surface. The outer piece is disposed around the inner piece and has an inner side surface, wherein the side surface of the inner piece is riveted to the inner side surface of the outer piece to form an interface, and an area ratio of the interface and the side surface of the inner piece is between 0.6 and 0.95. The light emitting diode lamp using the heat sink is also provided.