Abstract: The effects of MnCO₃ on grain of Mg-3wtAl and Mg-9wt% Al magnesium alloys were studied. The microstructure of as-cast Mg-Al alloys with MnCO₃ was analyzed by metallographic microscope. The results show that the average grain size of Mg-3wt%Al and Mg-9wt%Al alloys is 365 μm and 340 μm, respectively and the average grain size of Mg-3wt%Al and Mg-9wt%Al alloys decreases with increasing MnCO₃ addition.After the addition of 1wt%MnCO₃, the grain size is reduced to 90 μm and 55 μm, respectively. Theoretical analysis show that MgO, Al₄C₃ and Al₈Mn₅ particles are considered as heterogeneous nucleation particles and the grain size decreases obviously due to the increase of nucleation particles.