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Effects of calcium addition and electromagnetic stirring on the microstructure of AZ91 magnesium alloy and refinement mechanism were investigated. The results show that calcium addition ranging from 0.1 wt% to 0.3 wt% does not lead to formation of any new phases but cause the refinement of as-cast microstructure.However, combined calcium alloying and electromagnetic stirring significantly decrease the grain size, change the morphologies of the β-Mg17Al12 phases,and reduce their volume percentage. The minimum grain size of AZ91 alloy is obtained in the ease of the addition of 0.2 wt% Ca withexciting voltage of 100 V. The microstructural refinement is attributed to the increase of the degree of undereooling and nucleation temperature of primary α-Mg phases on the basis of DTA analysis results.