The effects of rolling reduction and rolling velocity on microstructure and mechanical properties of AZ31 Mg sheets were investigated in this paper. The grain size was significantly refined and the mechanical properties were raised after hot rolling. When the rolling velocity was 5m/min and the rolling reduction was 50%, the average grain size was refined to 9μm, and the tensile strength, yield strength and elongation were enhanced above 280MPa, 180MPa and 30%, respectively. The relationship between grain size and yield strength was studied. Based on the literatures on AZ31 rolling and experiments in this paper, the comparisons on mechanical properties of AZ31 Mg sheets were carried out. The results reveals that the rolling parameters used in this research can significantly enhance the mechanical properties of AZ31 Mg sheets, and the anisotropy in transverse direction and rolling direction was not obvious.