Hot rolling of AZ31 Mg alloy was performed by using as-cast alloy ingot as the starting material. The microstructures and mechanical properties of the as-rolled alloy subjected to various rolling passes were investigated. The results show that the grain size of the alloy can be refined steadily with increasing rolling passes by dynamic recrystailization. With the steady refining of the grain size, both the mechanical strength and the plasticity of the alloy are improved correspondingly. In particular, when the grain size is reduced to about 5 μm after 5 rolling passes, the yield strength, ultimate tensile strength and tensile fracture strain of the alloy are 211 MPa, 280 MPa and 0.28 in the transverse direction, they are 200 MPa, 268 MPa and 0.32 in the rolling direction, respectively.