This paper discusses a qualitative geometric reasoning method which approximately evaluates the equilibrium thermal characteristics of die-castings and dies at the conceptual design stage. The main idea is to apply circuit analog of heat flow on a voxel model and build the heat flow network for the die. An approximate temperature distribution on the die is then obtained by geometric reasoning and thermal resistance computation on voxel model. The resulting temperature field of the die is visualized using volume visualization techniques, and heat converging regions (hot spots) can be identified from the pattern. Thermal control features can then be placed according to the location of the heat converging regions and part thickness. A prototype system has been implemented on Microsoft Windows-NT platform.