Microstructure evolution of Sr modified Al-13Si-0.4Mg alloy during solidification process was observed by quenching and OM（optical microscope） to understand the un-uniformity of eutectic structure in the alloy.The results show that

during the initial stage of eutectic reaction

coarse eutectic structure is created in the alloy as a result of low branches in formed eutectic colony

characterized by thick eutectic Si flakes with wide lamellar spacing and a few branches.With advancing in reaction

eutectic grain is gradually refined due to the increase of branches in eutectic colony

characterized by thin eutectic Si flakes with narrow lamellar spacing and a lot of branches.Different enrichment of Sb at eutectic solid-liquid solidification front results in the different absorption coefficient of Sb on surface of eutectic Si

affecting the growth velocity of eutectic Si

which is responsible for the un-uniformity of structure in the alloy.