Grain refinement of magnesium alloys not only reduces the hot tearing and improves their mechanical properties, but also weakens the lexture and improves their isotropic properties. In aluminium-free magnesium alloys, the element Zr is an effective nucleants to refine the microstructure and its mechanism of grain refinement is clear. In Al-containing magnesium alloys, the grain refinement by carbon inoculation was considered to be the best approach until now. However, the mechanism of grain refinement is still unclear. After inoculated by carbon or SiC particles, the grain size of as-cast Mg-Al alloys reduces largely. Different from the previous results, a ternary carbide Al ₂MgC₂, which has a closer crystal structure to magnesium than A1₄C₃, was observed in Mg-Al alloys after inoculated by carbon or SiC particles. The mechanism of grain refinement by SiC inoculations is the same as that by carbon inoculation. The grain refinement is mainly attributed to the enhanced nucleation of alpha-Mg on the surface of the ternary compound Al₂MgC₂.