In this paper, the morphological character of AlMn intermetallics in Mg-3Al alloys and the mechanism of Manganese’s effect on heterogeneous nucleation of aluminum carbide (A14C3) were studied by using OM, SEM, EDS and DTA. Results show that A14C3 particles are produced by carbon inoculation and act as the potent heterogeneous nuclei for primary Mg. The ?-Mg grain can be greatly refined with carbon inoculation when the content of Mn is below 0.32wt%. The minimum average grain size of 66 μm can be obtained due to the formation of numerous small ball-like and short rod-like AlMn intermetallics, which is helpful to grain refinement. In contrast, coarser cross petals-like AlMn intermetallics will be formed if the content of Mn is increased to 0.56wt.%. Therefore, the A14C3 particles are easily poisoned by excess Mn solute and turn to be complex Al-Mn-C-O compounds, which are not effective heterogeneous nuclei of primary Mg, hence leading to the coarsening of ?-Mg grain.