A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (DAS), eutectic structure and fatigue behavior of two Al-Si-Mg casting alloys with silicon contents of 7% and 10% respectively, has been conducted. In the condition of minimizing casting defects, the influence of microstructural features on the mechanical performance becomes more pronounced. Depending on the magnesium content affecting the strength of the matrix, the tensile properties were changed upon experimental conditions; the tensile strength was increased with magnesium content, whereas the elongation was increased in the reverse case. The increase of both of high cycle fatigue and low cycle fatigue lives with decreasing the secondary dendritic arm spacing is observed, mainly due to homogeneous deformation owing to the fine size of eutectic silicon and Fe intermetallic particles. The fatigue dynamometer of a diesel cylinder head shows the same tendency with the results of specimen fatigue tests with microstructures.