Extensive research work was devoted to Mg-based alloys strengthened by precipitation hardening. In this framework, the Mg-Zn-Sn system was considered a promising candidate for a creep resistant Mg-alloy. Small additions of alloying elements forming high temperature phases (HTP) were used to improve the structural stability of the Mg-Zn-Sn alloy. Phase formation during solidification was analyzed using thermodynamic calculations. The influence of HTP-particles on stabilization of sub-grain boundary structure was found to be of great importance in improving structural stability of the alloys at elevated temperatures. Mechanisms of precipitation hardening were investigated using the modified Langer-Schwartz model calibrated for Mg-Zn-Sn alloys.