The influences of Ca addition (<1.2wt %) on structure and tensile fracture behavior of AZ91D alloy were investigated in this article. These results indicated that Ca addition can both form Al2Ca phase on interdendritic boundary and refine grain size. Ca addition evidently increases the yield strength but decreases the elongation of AZ91D alloy. The crack initiations of the AZ91D alloy and Ca-contained alloy both occur at the edges between the interdendritic eutectics and the α-Mg matrix. Failure of AZ91D alloy is brittle quasi-cleavage fracture, while Ca addition makes the failure tend to be cleavage.