Improved mechanical and thermal properties are important characteristics for enhancing the performance of cast iron components that operate at elevated temperatures. Thermal conductivity defines the temperature distribution within the casting and influences the magnitude of the thermally induced tensile stresses. The microstructural features that increase the thermal conductivity have a negative impact on tensile strength. The results reported in this work show that there is a unique inverse relationship between thermal conductivity and tensile strength, valid for the whole range of cast iron alloys regardless of graphite form, solidification rates, carbon content and matrix constituents. The finding indicates the challenges for the simultaneous improvement of these properties, and it can be utilized as a guideline during the design of cast iron components for high temperature applications.