We have investigated effect of Ca addition on the mechanical properties and aging hardening behaviors in Mg-Zn alloy. Mg-Zn and Mg-Zn-Ca alloys were fabricated under an Ar gas atmosphere. The molten metals were casted into the metallic mold, and cooling part. was located at the bottom of mold. The main interdendritic phase which formed during early solidification of the ternary Mg-Zn-Ca alloys is the Ca2Mg6Zn3 phase. The microstructure of Mg-6wtO/oZn-0.1similar to0.3wt%Ca alloys consisted of MgZn precipitates and Ca2Mg6Zn3 phase formed around the grain boundaries. In the alloys with the highest level of Ca(Mg-6wt%Zn-0.5similar to0.7wt%Ca alloys), the microstructure revealed wholly Ca2Mg6Zn3 phase formed around the grain boundaries. The grain size of Mg-6wt%Zn-Ca alloys decreased significantly with an increase in Ca content and, at 0.5wt% Ca or more, grain size becomes constant at 60mum. The tensile properties of the as cast Mg-6wt%Zn-Ca Mg alloys were improved due to grain refinement by addition of Ca. The Ca addition reduced the aging response of the Mg-Zn-Ca alloys compared with Mg-Zn binary alloy, and progressive additions of Ca to alloys decreased the amount of age hardening. The yield strength and ultimate tensile strength of the T6 treated Mg-Zn-Ca ternary alloys decreased with increase of Ca content, due to the brittle fracture arise from Ca2Mg6Zn3 phase formed along the grain boundary.