The effects of Al4C3 on the microstructure and performances of AZ91D alloy are studied by using SEM, EDS, EDS and DTA. It is found that a small amount Al4C3 addition to AZ91D magnesium alloy leads to the α-Mg grain size obvious decreasing, causes the eutectic microstructure morphology marked change from the fully-divorced β eutectic phase and lamellar α+β eutectic structure to honeycomb partially-divorced α+β eutectic structure, and the β-phases diminution of size and tendency to dispersed distribution. Based on the analysis of EDS, DTA and the calculation of disregistry between Al4C3 and Mg, it is found that the Al4C3 particles can act as the heterogeneous nuclei. The strength properties are significantly improved due to the microstructural refinement of AZ91D alloy, but a little change of elongation percentage, and slight increase of corrosion resistance