In this work, effects of the Sn additions (3 wt.%, 6 wt.%, 9 wt.%)on thermal conductivities of the as-cast, as-homogenized and as-aged Mg-Sn binary alloys were investigated. The results showed that thermal conductivities of the both as-cast and as-homogenized Mg-Sn alloys decreased with content of Sn addition, and corresponding thermal conductivity of the as-homogenized Mg-10Sn decreased as 52.6W/m.K. Compared with as-homogenized sample, thermal conductivity of the as-cast alloy with the same solute content was lower. Thermal conductivity of the Mg-Sn alloy increased with aging treatment, and thermal conductivities of the as-aged Mg-3Sn, Mg-6Sn and Mg-10Sn alloys saturated as 125W/m.K, 120W/m.K and 110W/m.K respectively. Purification of the Mg matrix contributed to the thermal conductivity increment, and atomic size, electron configuration and the charge of the Sn element determined the thermal conductivity of the Mg-Sn alloy.