The microstructure of reheated semi-solid A357 alloy billets in die casting was systematically analyzed by the quantitative method to understand the evolution and filling behavior of primary α phase.The results indicate that the primary α phase is coalesced and grown and spheroidized during reheating process with the increase of temperature and time.Coarse primary α phase is aggregated at the corner and at pouring position during filling process

while fine primary α phase appeares in the place with much liquid phase.