In a process for manufacturing a heat sink comprising a heat dissipating base plate and a multiplicity of tonguelike fins formed on an upper surface of the base plate, a heat sink blank of aluminum extrudate is formed which has a part for forming a thick portion of the base plate, a fin forming portion provided on an upper side of the thick portion forming part integrally therewith, a thin portion of the base plate provided at at least one of opposite sides of the thick portion forming part, and a level reference ridge formed on an upper surface of the thin portion and having an upper end at the same level as an upper surface of the thick portion to be formed. The fin forming portion of the blank is cut to a raised form at the same level as the level of the upper end of the reference ridge to form a multiplicity of tonguelike fins and form a thick portion of predetermined thickness under the fins.