The structural and hardness variations of Mg-5Sn-(1,2,3) Ca (wt%) alloys have been investigated during various aging heat treatments followed by solution heat treatment at $500^{circ}C$ for 24 hrs. Maximum hardness of Hv61 has been obtained for Mg-5Sn-3Ca alloys, when the aging treatment was performed at $200^{circ}C$ for 24 hrs. The microstructures were critically changed when the content of Ca was more than 1wt%, since CaMgSn and $Mg_{2}Ca$ phases were mainly precipitated during aging treatments. The hardness variations with structural evolutions are discussed with respect to the aging temperatures and times.