In this paper, further extrusion (Extrusion Ratio is 6.25) was adopted to commercial AZ31 magnesium alloy rods (Extrusion Ratio is 2) at different temperatures in order to refine the size of grain, then the rods with the crystallite size between 4-10 mu m was obtained, and the influence of crystallite with different grain size on the ductility of this alloy was investigated further. A lot of experiments and research indicated that: the elongation of the initial billet with the grain size of 30 mu m reached about 12%; after extrusion at different temperatures, the average grain size was refined to below 10 mu in, and the ductility rose up to 24%-30%. It also showed that, when the average grain size of AZ31 magnesium alloy was below 10 11 m, the ductility was effected obviously. Only when the average grain size was about a few microns, the ductility of AZ31 magnesium alloy was improved noticeably.