A heat sink has a heat-dissipating portion and a contact portion for contacting an electric device generating heat. The diameter of the contact portion is smaller than that of the heat-dissipating portion. The heat-dissipating portion has a plurality of fins. The contact portion is disposed on a bottom surface of the heat-dissipating and formed with the heat-dissipating portion as a single unitary member. The contact portion includes a first axial cross section and a second axial cross section. The first axial cross section is away from the heat-dissipating portion, and the second axial cross section is near the heat-dissipating portion. The diameter of the first axial cross section is larger than that of the second axial cross section.