Abstract: The effect of homogenization annealing on the microstructure of ZM21 magnesium alloy is investigated by means of optical microscope, SEM and EDS. The results indicate that there is segregation in the as-cast ZM21 alloy, and the solute elements such as Zn are concentrated at the grain boundary; the compounds in ZM21 magnesium alloy are decreased with the increase of annealing temperature,the better homogenization annealing process for ZM21 is at 693 K for 10 h. After the homogenization treatment, there are MgZn phase, MnZn₃ phase and Mn single phase in ZM21 magnesium alloy, in which Mn single phase is precipitated from supersaturated α-Mg solid solution, MnZn₃ phase is formed by the combination of Zn and Mn atoms through diffusion at a high temperature for long time.