Mg-Sn alloys are promising for the development of new cheap creep resistant magnesium alloys. In the present paper, the creep behaviours of Mg-Sn and Mg-Sn-Ca alloys were examined at the constant temperature and different stresses. The measurements of stress exponents indicate that the dislocation climbing is the dominant mechanism during the creep of Mg-3Sn or Mg-3Sn-2Ca alloys. The poor creep resistance of the binary Mg-3Sn alloy is caused by the easy movement of dislocation and the segregation of Sn at the boundaries. Both T4 and T6 heat treatments improve the creep resistance of Mg-3Sn alloy due to the alleviation of Sn segregation at grain boundaries and the precipitation of Mg2Sn particles, respectively. Ca is an effective alloying element to increase the creep resistance of Mg-Sn alloys. The Ca addition leads to the formation of thermal stable phases Mg2Ca and CaMgSn in Mg-3Sn-Ca alloys. These two phases effectively hinder the movement of dislocations and the sliding of grain boundaries. On the other hand, the addition of Ca alleviates the segregation of Sn by the interaction of Ca with Mg and Sn to form the phase CaMgSn.