The effects of Ca addition on the microstructure and mechanical properties of the extruded AM50 magnesium alloy have been studied. The results show that the Ca addition can reduce the quantity of Mg17Al12phase and form a new A12Ca phase distributing along grain boundaries. When Ca is added with an amount of more than 2wt. %, needle -shaped Al2Ca precipitates are observed inside grain interiors, and the grain sizes of both as -cast and as - extruded AM50 alloy are decreased from 200μm and 15μm to 40μm and 7. 5μm, respectively. It is found that the yield strength of the extruded AM50 alloy are improved with increasing Ca content in the studied temperature range, while the improvement in ultimate strength can be observed only at low temperature.