The mechanical properties, microstructures, and deformation behavior for a cast Al-7Si-Mg casting alloy A357 under various aging conditions have been investigated in the present paper. It is shown that the combination of strength properties and elongation varies with aging parameters. Aging at 165��C gives the optimum balance of strength and ductility. Transmission electron microscopy (TEM) observation shows that, in the underaged conditions in which Guinier-Preston (GP) zones are formed, the tensile deformation involves dislocations, which cut through the GP zones. Planar slip bands can be seen in the deformed substructures, and the slip bands coarsen with increasing aging temperature or time. In the overaged conditions in which �� phases precipitate, the dislocations bypass precipitates and distribute uniformly, except some dislocation tangles around coarser precipitates. When the interaction mode between dislocations and particles changes from shearing to bypassing, the deformation homogeneity greatly increases for the castings with fine �� precipitates, which may explain the significant secondary elongation behavior. Based on these studies, a step overaging treatment has been used to improve the overall tensile properties.