Microstructural evolution of Mg-Sn-Ca system depending on the Ca/Sn ratio and their high temperature properties have been investigated in the present study. As-cast microstructures of the Mg-Sn-Ca alloys exhibit typical dendritic structure consisted of primary α-Mg and secondary solidification phases. The secondary solidification phases have been identified as Mg2Sn, CaMgSn or Mg2Ca depending on the Ca/Sn ratio. With small amount of Ca addition (∼1wt%), eutectic Mg2Sn phase disappears and CaMgSn phase is newly formed. Eutectic Mg2Sn phase at grain boundary deteriorates the creep resistance. On the other hand, Mg2Ca phase remarkably improves the creep properties. This result is attributed to the inherent thermally stability and precipitation of the Mg2Ca phase in matrix.