Si modification behavior by twin probability approach with X-ray diffractometry (XRD) technique and microstructural evolution of the A356 alloys with Al2Ca and Mg exceeded up to 1.5 mass% during the solution heat treatment were investigated. Al2Ca added alloys showed the modified Si particles in the microstructure and higher twin density than those of Al2Ca-free alloy. During the solution heat treatment, the Si particles of Al2Ca-free alloy became coarser with the time at 540 °C, while there was no coarsening in the Al2Ca added alloys throughout all the conditions.