The structural stress and fatigue behavior of tensile specimen containing internal shrinkage defect were modeled. Real shrinkage defect in casting was scanned by industrial CT (computed tomography), and subsequently its shape was simplified by ellipsoidal primitives for the structural analysis (S.S.M., shape simplification method). The analysis results were compared with the results by real shrinkage shape without any simplification process. It was possible to consider real shrinkage of casting in stress analysis and the method to predict fatigue life of casting with defect was proposed.