Abstract: Magnesium was added into the Zn-35%Sn alloy to study the influences of magnesium content on the microstructure and mechanical properties of the alloy, and the tensile fracture morphology was analyzed. The results show that trace amounts of magnesium could make significant influence on the microstructure and mechanical properties of Zn-35%Sn alloy. The magnesium added into Zn-35%Sn alloy partially dissolved into the Zn-rich η phases, making the growth of atoms in this phase along the direction perpendicular to close-packed side and forming rod-like microsturcture. The other part of magnesium reacted with Stannum and generated Mg2Sn which could be used as an effective heterogeneity nucleus of γ-Sn phase, impeded it transforming into β-Sn and reserved brittle microstructure. With the increase of magnesium content, the size of η-Zn phases increased, grain-orientation was more obvious while cleavage fractures became large, the plasticity of alloy dramatically reduce.