Within this research work the fatigue properties, fracture range and crack propagation of case hardened spheroidal cast iron (S.C.I./ADI) in the ausferritic state have been investigated in comparison to non hardened ausferritic castings. Depending on austempering conditions microstructural features partially associated with martensitic reactions are obtained which influence strongly the mechanical properties. Under cyclic push-pull-load fatigue strength of specimens of austempered spheroidal cast iron have been compared with additionally induction hardened samples. Influences on crack formation due to the shape of samples, strengthening parameters concerning different heat treatment conditions have been determined with metallographic and electron probe analytical methods. Influences on the crack propagation by structural changes of retained austenite and ferrite, and also the shear induced martensite transformation enhanced by the fatigue notch factor have been investigated. SEM and TEM analyses confirmed the deformation in retained austenite by twinning lamellae and dislocation structures in the adjacent ferrite sheaves. Quantitative data of the crack propagation velocity have been determined on ADI spheroidal cast and compared with untreated S.C.I. Slight, but significant retardations in crack velocity due to austempering have been detected.