Fracture samples were prepared from different positions in practical fatigue failure ADC1 castings

and surface morphology and characteristics of the fracture samples were analyzed by SEM （scanning electron microscope） and EDS （energy dispersive spectrometry） to understand origination of fatigue failure of the castings. The results show that there exists pore with a larger size near surface and congregation of large area oxide film in the casting

which are responsible for the fatigue failure of the castings. In addition

the role of pore and oxide film in the process of failure was described.