Effects of Mg-50 Al4Ce3 master alloy and Ce on the microstructural refinement of AZ31B magnesium alloy were observed by SEM （scanning electron microscope）,EDS （energy dispersive spectrum） and XRD （X-ray diffraction）. The results reveal that dendrite and grain size in AZ31B magnesium alloy can be remarkably refined with 0.5% Al4Ce3,and it is further refined with 0.5% Al4Ce3 and 0.3% Ce mixed addition,in which average grain size of α-Mg is decreased from 280μm in matrix alloy to approximately 50μm,and β phase is converted from continuous network structure into discontinuous network structure and fine particle with generation of Al4Ce phase. Based on the analysis of EDS and calculation of the planar mismatch between Al4Ce3 anf Mg,Al4Ce3 particles can be acted as the heterogeneous nuclei of primary α-Mg phase. Ce can result in the increase of constitutional undercooling of alloy,which can activate potential Al4Ce3 to form nucleation at solid-liquid interface front to increase the nucleation rate.