Both master alloy and die casting technology can refine the grain size of primary Si in hypereutectic Al-Si alloy in die casting. The effects of Al-P master alloy on the microstructures of hypereutectic Al-5Si alloy in die casting and in metal mold casting were approached. It is revealed that compared with microstructure of hypereutectic Al-Si alloy without Al-P master alloy modification in metal mold casting

the grain size of primary Si phase without Al-P master alloy modification is reduced by 50% while with Al-P modification is decreased by 75% in die casting. Compared with that of the alloy without Al-P modification in die casting

the primary Si size with Al-P modification in die casting is decreased by 40%. It is necessary to modify the hypereutectic Al-Si alloy by Al-P modifier in die casting.