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The effect of Si (individual and combined) with 0.2 pct Sb additions on the microstructure and mechanical properties of permanent mold AZ91 alloy has been studied. The results indicate that Si addition introduces a Chinese script Mg 2 Si phase at the grain boundary along with the Mg 17 Al 12 -␤ phase and reduces ductility and strength of the alloy both at room and high temperatures. A small amount of Sb addition modifies the Mg 2 Si phase besides distributing it evenly along the grain boundary. Improved room-and high-temperature mechanical properties are observed in Sb added AZ91 ϩ xSi alloys. However, maximum properties are noticed with the alloy having the combined addition of 0.2 silicon and antimony. Fractograpy of tensile-and impact-tested AZ91 alloy shows cleavage and brittle type of failure. Addition of Si reduces the quasi-cleavage planes, whereas Sb addition increases it while also increasing the plastic zone.