Stress strain behavior at the notch root under cyclic loading was investigated with finite element analysis for 2024-T4 aluminum alloy. Results of the analysis indicated that plastic deformation occurred at the notch root and stress ratio at the notch root decreased with increase in applied stress amplitude level. Fatigue tests were carried out with smooth and notched specimens. The experiment results showed transition behavior of S-N curve in notched specimen. According to result of finite element analysis, decrease in stress ratio induced by local plastic deformation at the notch root might affect the transition behavior of S-N curve in notched specimen.