Abstract: Using the optical microscopy, scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and tensile tesing, the microstructure and tensile properties of AZ91D magnesium alloy compositely added rare earth Nd and Er were studied. The results show that the microstructure of AZ91D alloy was obviously refined, and a quantity of granular rare-earth compound in grains and precipitates along grain boundaries were formed after the alloy was compositely added rare earth Nd and Er and then was treated by 420 ℃×8 h solid solution and 165 ℃×10 h artificial aging. When 0.8wt.% Nd and 0.4wt.% Er were added into the AZ91D alloy, the grain size of the AZ91D alloy reached the minimum, the uniformly dispersed precipitates in the grains reached the most in quantity, the continuous network precipitates along grain boundaries were formed, the tensile strength of the alloy at room temperature and high temperature reached the maximum value, 238 MPa and 182 MPa, which were increased by 30% and 21% compared with the AZ91D alloy added 0.4wt.% Er only.