The superior combination of attainable properties has caused austempered spheroidal graphite cast iron to emerge as a new class of cast iron. The metallurgy of austempered spheroidal graphite cast iron is presented with the purpose of clarifying the mechanical properties. It is emphasized that segregation of alloying elements can cause an iron to behave differently than might be expected. The role of the graphite nodule count on the strength and the toughness of this iron is discussed from the standpoint of the matrix structure, because segregation can be controlled by varying the nodule count, i.e. solidification rate during casting process. High carbon-reacted austenite increases with increasing nodule count, but instable unreacted austenite decreases. With increasing nodule count, the tensile and fatigue properties are improved, but the elasto-plastic fracture toughness decreases. This behaviour is explained without contradiction by the microstructures including the reacted and unreacted retained austenites and the notch effect of the graphite. The austempered spheroidal graphite cast iron has very desirable characteristics for a wide range of applications as component parts of automobiles, construction machinery, railroad, etc.