Fatigue fracture of the semi-natural cast iron plate with dross and carbon float defects was analyzed. Crack formation and propagation were traced under the programmable cyclic load of the pure bending. The focuses of the crack and their consistent pattern up to the total fracture were determined. Fracture analysis was performed by using optical and electronic microscopy methods. High-cycle fracture mechanism was established when the number of the cycles exceeded 108. The influence of the microstructure upon fracture process was investigated.