In the present paper, thermal optimization of a plate-fin heat sink with the fin thickness varying in the direction normal to the fluid flow was conducted. The model used for this optimization was based on the volume averaging theory (VAT). It was shown that the thermal resistance of the plate-fin heat sink can be reduced by allowing the fin thickness to increase in the direction normal to the fluid flow. In the case of a water-cooled heat sink, the thermal resistance decreases by as much as 15%. The amount of the reduction increases as the pumping power increases or as the length of the heat sink decreases. (C) 2010 Elsevier Ltd. All rights reserved.