The influences of small amount of Sn addition on microstructure and creep resistance of AZ91-0.4%Ca alloy have been investigated. The microstructure of the AZ91-0.4%Ca alloy was characterized by ${alpha}$-(Mg) dendrite cells surrounded by eutectic ${eta}(Mg_{17}Al_{12})$ and $Al_2Ca$ phases. The 0.5%Sn addition resulted in the formation of rod-shaped CaMgSn particles with the extinction of $Al_2Ca$. The Sn-containing alloy exhibited better creep resistance below $175^{circ}C$, but the tendency was reversed above $200^{circ}C$. The reason was discussed in relation to the change in thermal stability of ${eta}$ phase in response to the Sn addition.