Abstract: The microstructure, high temperature mechanical properties and the precipitation phase of Mg-12Gd-3Y-0. 5Zr alloy under different states were investigated by using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and high temperature tensile testing machine. The results show that the as-cast alloy was consisted of α-Mg solid-solution and Mg5Gd precipitation phase and α-Mg+Mg24Y5 eutectic. The grains size of the alloy decreased obviously after hot extrusion. The extruded and rolled sheets had fairly high tensile strength at room temperature and 150 ℃, and the strength decreased rapidly as the temperature raised further. The aging precipitate phases of the alloy sheet did not change in the process of high temperature (＞250 ℃)extruding, but the distribution and morphology were changed, which led to the decrease of deformation resistance.