The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been studied. The results show that the Ca addition can refine the microstructure, reduce the quantity of Mg17Al12 phase, and form new Al2Ca phase in AZ91 magnesium alloy. With the Ca addition, the tensile strength and elongation of AZ91magnesium alloy at ambient temperature are reduced, whereas Ca addition confers elevated temperature strengthening on AZ91 magnesium alloy. The tensile strength at 150 degreesC increases with increasing Ca content. The impact toughness of AZ91magnesium alloy increases, and then declines as the Ca content increases. The tensile and impact fractographs exhibit intergranular fracture features, Ca addition changes the pattern and quantity of tearing ridge, with radial or parallel tearing ridge increasing, tensile strength, elongation and impact toughness reduce.