The microstructural evolution and mechanical properties of as cast AZ91 Mg alloy enduring different homogenization conditions were researched. The results show that the peak stress and plasticity obtain improvement with the increase of heat treated time and temperature until 693K. When the temperature is higher than 743K, the sample will occur the over-burned and directly lead to the decrease of mechanical properties. Otherwise, with the increase of heat treated time and temperatures, the second phase gradually precipitate from the solution, while the grain evolution is not obvious. In the compression process for the samples after heat treatment, some twins could be found in the grains.