Abstract: With the help of optical microscope(OM),scanning electron microscope(SEM),energy dispersion spectrometer(EDS) and X-ray diffraction(XRD) effect of EMS on microstructure of AZ91D magnesium alloy alloyed with Ce-rich MM has been investigated in the present paper.The results showed that with the addition content of Ce-rich MM increasing primary-Mg grains are refined,the content of β(Mg₁₇Al₁₂) whose morphology evolves from continuous network to detached sphere decreases and its size decreases,the amount of new formed Al₁₁MM₃ increases so much that the distribution transforms form short-rod in grains and at grain boundaries to fine acicular assembling at grain boundaries in the form of cluster.Under the impact of strong EMS the lower the isothermal stirring temperature is,the finer the primary α-Mg grains are,furthermore morphology primary α-Mg evolves from sphere with a few rosette to sphere-like and polygon;and Al₁₁MM₃ distributes in grains and liquid phase in the form of sphere-like and fine acicula respectively.Ce-rich mish metals results in the refinement of grain,content decrease of β phase and morphology and distribution melioration of β phase.Morphology and distribution of Al₁₁MM₃ vary with content of Ce-rich MM.EMS makes the primary α-Mg and the second phase Al₁₁MM₃ fine and spheric.