Abstract: The Al-30wt% Mg₂Si composites were made by in-situ process and modified by adding rare earth element yittium (Y) with different contents, in order to investigate the effects of Y on microstructure and mechanical properties of composites and the refinement mechanism of Y on Al-30wt% Mg₂Si composites. The results show that, the average grain size of primary phase Mg₂Si decreases firstly then increases; but the strength, Brinell hardness and elongation increase firstly then decrease, when the addition amount of Y increases from 0 to 0.8wt%. With 0.6wt% addition amount of Y, refining effect of Al-30wt% Mg₂Si composites is most obvious and the mechanical properties increase observably. Tensile strength increases from 145.5 MPa to 175.2 MPa, yield strength increases from 128.8 MPa to 152.0 MPa, Brinell hardness increases from HB101.4 to HB129.4, elongation increases from 2.4% to 3.3%.