AM60B alloy has been refined by SiC particles and the corresponding refining mechanism has been mainly discussed. The results indicate that the addition of 0.2 wt% SiC particles in form of mixture with Mg powder decreases the grain size from 317 mu m of the not refined alloy to 46 mu m. The decrease of beta phase and formation of Mg2Si and Al4C3 phases well demonstrate that the reactions of 3SiC + 4Al = Al4C3 + 3Si and 2Si + Mg = Mg2Si occur during refining treatment. In addition, the crystal nuclei are composed of two kinds of elements, Al and C. All of these imply that the formed Al4C3 particles are the actual heterogeneous nucleation substrates.