The tensile creep performance of Mg–Sn alloys, containing Zn and Zn+Na alloying additions, have been measured at temperatures of 177 and 150°C. Under a load of 60MPa, the Mg–Sn–Zn–Na alloy shows secondary creep rates five orders of magnitude slower than the binary alloy and approximately three orders of magnitude slower than the ternary Mg–Sn–Zn alloy. Transmission electron microscopy shows that the origin of the excellent creep properties of the Mg–Sn–Zn–Na alloy is the remarkable thermal stability of the precipitate structure.