Typically, the transient thermal error accounts for up to 70 percent of the total error in machine tools and precision devices. To decrease the effect of thermal distortions, design optimization is used as tool to modify, e.g., the material thickness, structural shape and cooling. This paper presents a heuristic design optimization method relying on thermal modes. The method reduces the system using the most relevant modes, which are identified using three designer-based response functions. Although the method may not converge to the global optimum and may need some tuning of inputs, it results in manufacturable designs. In addition, the designer-based response functions give insight in the design. Thus, this heuristic optimization method is a good way to reduce the thermal error in the design of a thermo-mechanical system. Keywords: thermal error, thermo-mechanical, heuristic design, design optimization, thermal modal analysis@en