Abstract: In-situ Mg₂Si/Al composites were prepared by common weight casting technique and the rare earth Y was selected. The change of the morphology of primary phase Mg₂Si, the size and mechanical properties because of rare earth Y content was investigated. The results indicated that there were the effects of Y on the microstructure of Mg₂Si/Al alloy composites, and Mg₂Si particles treated by Y in the composites became finer. By calculating the planar disregistry, it is found that the Y can be used as the heterogene-nuclei of the primary Mg₂Si phase to refine grain size, in which Al₃ Yphase formed by interaction between Y and Al could hold back Mg₂Si phase growth. The mechanical properties of the composites with addition of Y were improved, in which the maximum tensile strength and elongation of the composites reached 144MPa and 4.19%, respectively.