Abstract: The effect of speed ratio, rolling reduction ratio and annealing on the recrystallization grain size of AZ31 magnesium alloy sheets was studied under the condition of differential speed roiling process. It is shown that the grains can be significantly refined by increasing the speed ratio and reduction ratio. But when the speed ratio is too high, sliding occurs between the rolls and sheets, lower-ing grain refinement. A speed ratio of 1.3 provides the best grain refinement. The major reason that differential speed rolling leads to a better grain refinement is that shearing deformation in differential speed rolling can increase the number and uniformity of twins and cut up dislocation cells or walls, the distortion energy more evenly distributes in the magnesium alloy. In addition, a reasonable differential speed rolling process also reduces the middle and final annealing time.