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Mg-50%Al 4 C 3 -6%Ce master alloy has been prepared by powder in-situ synthesis process. Phases compositions, micro-morphology and micro-area composition were analyzed by XRD (X-ray diffraction), SEM (scanning electron microscope) and EDS (energy dispersive spectrometer). The results show that the master alloy mainly contains lamellar Al 4 C 3 , rod-shaped Al 4 Ce and Mg. The microstructure of AZ91D alloy is obviously refined after Mg-50%Al 4 C 3 -6%Ce master alloy addition. The dendritic morphology of α-Mg evolves from a characteristic six-fold symmetrical shape to a petal-like shape. After adding 1.2wt% master alloy to AZ91D magnesium alloy, the average grain size is obviously decreased from 360 to 65 μm. The mechanism of grain refinement is ascribed to the Al 4 C 3 particles acting as heterogeneous crystal nucleus of primary α-Mg and Ce can activate the potential Al 4 C 3 particles on the solid-liquid interface.