A356 aluminum alloy has excellent casting ability, corrosion resistance and good strength to weight ratio especially after proper applications. These properties make Al-Si alloys an ideal material for the automotive and aerospace industries. In this study, we investigated the correlation between the solidification structure and mechanical properties A356 alloy during heat treatment. The microstructural showed the eutectic silicon and alpha Al phase in the A356 alloy castings depending on the amount of strontium are investigated during solution treatment using OM, SEM and TEM. Also, the tensile tests examined the relationship between the microstructure and the fracture surface. The as-cast A356 alloys under 40ppm strontium showed undermodified microstructure, however, that of the added 60~80ppm strontium have well modified structure of fine fibrous silicon. After solution treatment, the microstructure of the undermodified A356 alloy exhibited partially a spheroidized morphology, but the remains described the fragmentation of fibrous silicon.