<p>Addition of the 2% Sn effect on the microstructure and hardness of AZ91 magnesium alloy in the as-cast condition were systematically investigated. First, the AZ91 alloy and AZ91-2%Sn alloys were synthesized in the protective atmosphere vacuum-assisted furnace. Experimental results revealed that the as-cast AZ91 alloy contained of two primary phases: α-Mg and β-Mg₁₇Al₁₂, while the microstructure of AZ91-2%Sn alloy consists of an additional Mg₂Sn phase along with α-Mg and β-Mg₁₇Al₁₂. Microstructural analysis (optical micrographs and FESEM) showed that the 2% Sn addition led to significant grain refinement and the transformation of partially divorced β-Mg₁₇Al₁₂ eutectics into fully massive β-eutectics. These massive β-eutectics and Mg₂Sn phases in AZ91-2%Sn alloy enhance the mechanical strength of the alloying added material as compared to base alloy. The Vickers hardness value of the AZ91-2%Sn alloy was increased to 132 ± 5 HV as compared to AZ91 75 ± 5 HV due to brittle behavior of fully massive β-eutectics and thermally stable Mg₂Sn phase.</p>