Microstructures and mechanical properties of Mg-6Al-1.2Y-0.9Nd magnesium alloy with Sn addition were investigated by OM, SEM, EDS and XRD. The results show the addition of Sn can dramatically enhance the tensile strength of the alloy in the temperature range of 25-175 oC. When the content of Sn is up to 1%, the values of tensile strength of the alloy at room temperature and 175 oC are up to their maximums simultaneously, 242 MPa and 192 MPa, respectively. The fracture of the alloy is quasi-cleavage failure with plastic characteristic. It can be found that the grain of alloy is obviously refined with the addition of Sn. The high-melting phases Mg2Sn is found in the alloy with the addition of Sn. Sn can improve the mechanical properties of Mg-6Al-1.2Y-0.9Nd alloy because of the grain?refining?strengthening, solution strengthening and dispersion strengthening.