In the presence of casting defects, aluminum components are subjected to multiaxial loadings. The objective of this study is the estimation of the fatigue limit for defective cast aluminum alloy A357 with T6 post heat -treatment. This investigation is based on the analysis of the Crossland equivalent stress in the vicinity of the casting defect using the finite element (FE) method. The defect shape is a semi-spherical void at the surface of a sample submitted to tension fatigue loading. The affected depth of this material is calculated and defined as the depth from the tip of the defect to the interior of the specimen. The Kitagawa-Takahachi diagrams corresponding to different microstructures and two loading ratio R=- 1 and R=0 are simulated based on the affected depth approach. Simulations were compared to the experimental results carried out on the same alloy. The calculations agree well with results inferred from experimental results. Finally, a comparative analysis is carried out between A356 and A357 built on the affected depth approach.