In most spheroidal graphite cast iron, tiny casting defects such as abnormal graphite, micro shrinkage, and inclusions, etc. occur near the casting surface. These defects deteriorate the fatigue strength depending on the size and distribution. Since the defects size is mostly within a range of 100 to 1000 μm, it is difficult to detect them in the production process. In this study, ferritic-pearlitic spheroidal graphite cast iron specimens were prepared with casting surface and machined surface. Then, a simple measurement of the defects size was carried out by microstructure observation or low temperature fracture testing. The estimated value of the maximum defect size could successfully be evaluated based on the statistics of extremes analysis for both micro shrinkage and dross defect near the casting surface. Moreover, four-point bending fatigue tests were performed at an R of 0.3 to investigate the influence of the defects near the casting surface on the fatigue strength. The S-N data showed a scatter because of the influences of the different defect sizes in respective specimens. The influence of the defects on the fatigue limit could be predicted by both statistics of extremes analysis of the defect size and the √area_max parameter model. Several examples will be also given on how to apply these methods for designing ductile cast iron products.