In the last few decades, the number of the aging aircraft has increased steadily. As the aircraft aged, the corrosion and fatigue cracking which cause the degradation of flight safety are increased. In this study, fatigue lives of the specimens from aging aircraft were experimentally confirmed and these were showed lower, comparing with the pristine data. The major factors causing the fatigue life difference between the exposed and the pristine were studied based on the morphology of the fracture surfaces. It was found that multiple cracks were initiated at the defects on the surface of the exposed specimens. The major defect which caused the failure during the fatigue test was measured and analyzed to convert into Equivalent Initial Flaw Size (EIFS) to predict fatigue life using a commercial code (NASGROW). The predicted fatigue lives were compared to the experimental results to know the fatigue cycles required to develop EIFS to the fatigue crack.