Abstract: AZ31 magnesium alloy ingot was self-prepared, extruded and annealed at 350 ℃ for different times in order to obtain the alloy sheet containing initial grains with different sizes, and then the sheet was rolled with reductions of 40% and 80%. The effects of initial grain sizes on microstructure and mechanical properties of the rolled sheet were investigated. The results show that the microstructure was significantly refined after large strain rolling with reduction of 80%, the main recrystallization mechanisms were twining induced dynamic recrystallization and rotation dynamic recrystallization. The role of grain rotation was restrained while the role of twinning was enhanced with the increase of initial grain sizes, therefore the role of twining induced dynamic recrystallization dominated, consequently, the homogeneous recrystallization microstructure and good mechanical properties were obtained. When the reduction was 80%, the tensile strength, yield strength and elongation of the magnesium sheet with the largest initial grain size of 5 μm were 311.3 MPa, 206.8 MPa and 28.3%, respectively.