A heat sink includes a base member having stepped channels spaced on the surface thereof in a parallel manner and first and second ribs protruding from the surface and respectively extending along two opposite lateral sides of each of the stepped channels, and radiation fins respectively mounted in the channels of the metal base member and supported on the second ribs vertically, each radiation fin having a Z-shaped foot portion that is inserted into one respective stepped channel of the base member and secured thereto by the associated first and second rib that are stamped to clamp on the Z-shaped foot portion of the associated radiation fin after its insertion into the respective stepped channel.