The La2(CO3)3 addition makes the grain of AZ91 alloy be refined obviously.The average primary dendrite size of the alloy decreases from 150 to 50 μm.The microstructure of typical dendrite turns into the equal-axed grain with the addition of 2.5 wt.% La2(CO3)3 refiner.The mechanical properties tests indicate that the tensile strength and especially elongation of the alloys are improved with the addition of La2(CO3)3 and their maximum enhancing rates based on the La2(CO3)3-free AZ91 alloy are 26% and 5.2 times at 2.5 wt.% La2(CO3)3 refiner respectively.