In this study, effect of Sn addition on microstructure, mechanical and the casting properties of AZ91 magnesium alloy have been studied. Results from the microstructural analysis showed that refinement of Mg17Al12 phase took place and new Mg2Sn phase was formed as Sn was added into the AZ91 alloy. Fluidity increased with 0.5 wt.% Sn content then decreases rapidly as the Sn content exceeded above 0.5 wt.%. Hot tear susceptibility (HTS) decreased with 0.5 wt.% Sn addition to AZ91 magnesium alloy above which it increased considerably. Tensile strength and elongation increased by addition of up to 0.5 wt.% Sn above which it decreased with increasing Sn content. The increase of mechanical properties was attributed to transformation of lamellar to fully divorced eutectic beta phases by 0.5 wt.% Sn addition existed in grain boundaries of the magnesium matrix. The reduction in the mechanical properties above 0.5 wt.% Sn was attributed to formation of clustered Mg2Sn phase. (C) 2013 Elsevier Ltd. All rights reserved.