Abstract: An investigation on the fracture mechanism of three stage of fatigue crack propagation for nodular cast iron with different percentage of pearlite and ferrite has been carried out by means of electron fractographic analysis and metallugical microscope observation for fatigue crack propagating path. The following results are observed.The fatigue crack initiation and propagation occur preferentially on the boundaries betweengraphite nodule and matrix. The crack initiation and propagation are effected by both theshape and distribution of spheroidal graphite.In both the crack initiation and the early stage of fatigue crack propagation. The fracture offerriteris characterized by cyclic cleavage, and the fracture of pearlite is characterized by acombination of cementite cleavage and fatigue striation in ferrite. The rate of early stage ofcrack propagation are controlled by△K (th). The number of stress cyclic for initiation of fatigue crack No are controlled byσs and△K (th). No is governed by△K (th) in blunt nottched specimen,No is governed by△K (th) in case of sharp notches.In the stable stage of fatigue crack propagation, the ferrite is characterized by fatigue striation. The pearlite is characterized by fatigue striation and cleavage, the bull's eye ferrite of 80% pearlite nodular cast iron is characterized by fatigue striation and shear region. The fatigue striation spacing able to reflect variation of da/Dn. In fast stage of fatigue crack propagation, the fractographic is characterized by a combination of fatigue striation and static fracture. The rate of fast fatigue crack propagation is governed by fracture toughness.