Specimens of as-cast Mg-1Ag-Zn, Mg-3Ag-Zn and Mg-5Ag-Zn alloys were fabricated, and then the specimens’ thermal conductivity was calculated. With the increase of Ag content, the thermal conductivity decreased significantly of as-cast alloys. After that, directional solidified specimens of the above mentioned as-cast Mg-Ag-Zn alloys were made at three different pulling velocities: V=25μm/s, V=50μm/s, V=75μm/s, and then again calculated their thermal conductivity. The result showed that with the increase of the pulling velocity, the thermal conductivity of alloys decreased significantly. Variations in the solute content and the pulling velocities are believed to play important roles on the thermal conductivity of Mg- Ag-Zn alloys. Since those two circumstances can strengthen the electron’s scattering process, reduce their free path, and thereby reduce the alloys’ thermal conductivity.