Casting thick-walled parts increases the risk of getting discontinuities in the microstructure, although the casting process is well-configured. Such discontinuities are, e.g., abnormal graphite, dross, shrinkage and microporosity. In this diploma thesis the influence of chunky-graphite on the ultimate tensile strength and on the fatigue behaviour has been studied. Furthermore, the microstructure of the casting part in regions with and without chunky-graphite has been characterized by metallographic methods and image analysis. Investigations on the microstructure of thick-walled cast parts show an inhomogeneous distribution of the spheroliths in size, density and shape and a varying percentage of chunky-graphite over the cross-section of the cast component. This diploma thesis evidences that, chunky-graphite, basically, deteriorates the mechanical properties of nodular cast iron. If chunky-graphite appears in the microstructure it diminishes tensile strength, fracture strain and the endurance limit. The impairment of the properties of results of the static tensile tests and dynamic fatigue test do not correlate and are strongly dependent on the percentage of chunky-graphite-cells in the microstructure. This is the reason why satisfactory lifetime prediction of thick-walled cast components demands an implementation of the fraction of casting failures in the microstructure and its deterioration of the mechanical properties. This work improves the understanding of the graphite formation in thick-walled cast components and its mechanical properties.