In this paper, the influence of the coarsening treatment of eutectic Si particles on fatigue strength and fracture toughness of a peak-aged Al-7%Si-0.4%Mg alloy casting has been investigated. By the coarsening treatment at 808°K for 100 hours, the eutectic Si particles grew to be spherical and were uniformly distributed, but casting defects remained without any changes. On tensile tests, the 0.2% proof stress and tensile strength were almost the same in both as-received material and Si-coarsened material, but the ductility was much higher in the Si-coarsened material than in the as-received one. Fatigue fracture occurred by crack initiation from the casting defects and succeeding crack growth on both materials, and the fatigue strength of the materials was affected by the size of defects rather than by the coarsening treatment. On the other hand, the fracture toughness was distinctly improved by the coarsening treatment. It is suggested that the improvement of fracture toughness in the Si-coarsened material is attributed to the energy dissipation due to the damage of Si particles around a crack and the rough fracture path.