This paper shows that the hot spot temperature of an electronic module with finned air heat sink can be reduced by allowing the fin thickness and height to increase in the flow direction x. The hot spot temperature decreases by about 15% if the thickness of a plate fin with constant height increases as x0.42. The decrease is approximately 30% if the height of a constant-thickness plate fin increases as x, i.e. if the fin shape is almost like a triangle when viewed from the side. In addition to lowering the hot spot temperature, the fin thickness and height variations recommended by this study lead to considerably more uniform temperature distributions on the module surface on which the fins are installed.