Abstract: In-situ formed Mg2Si/AZ91D composites were fabricated by adding Al-Si alloy into the AZ91D magnesium alloy.The effects of calcium and strontium on the as-cast microstructure and mechanical properties of composites were investigated.The results indicate that adding Si to AZ91D magnesium alloy could form Mg2Si strengthening phase of large size with high melting point and high hardness.The combined addition of calcium and strontium refined the as-cast microstructure of composites significantly,and also improved the morphology of Mg2Si particles,so the mechanical properties of composites were increased.