Abstract: Al-24%Si alloy was modified with cerium-rich mischmetal.The morphology and size of the Si phase were studied by using SEM,and the modification effect and mechanism were analyzed.The results show that after modification both primary Si in the alloy and eutectic Si had been well refined.The average size of primary Si could be reduced from 100 μm to 50 μm,and the eutectic Si could be transformed into granular or short rod.Cerium and oxygen could occur chemical reaction,and the product was CeO2,which could become the heterogeneous nuclei of primary Si,so as to produce numerous Si crystal nuclei.Eutectic Si was refined by the forming of cerium-rich layer at the growth interface,which led to the composition undercooling and reduce the growing speed of eutectic Si.