The cast AZ31 magnesium alloys were treated by a single-pass friction stir processing (FSP), and the microstructure and mechanical properties of the processed region were studied. The results show that severe plastic deformation occurs and the coarse eutectic β-Mg17Al12 phase was broken, forming a refined and homogeneous dynamic recrystallized microstructure during the FSP period. XRD patterns indicate that a large amount of β phase dissolve into the magnesium matrix because of FSP, resulting in solid solution strengthening. The average microhardness (HV) of the FSPed AZ31 sample is 810 MPa higher than that of the cast AZ31 alloy, and the ultimate tensile strength and elongation of FSPed sample increase by 43 MPa and by 4.3% compared with those of the cast AZ31 alloy, respectively. Moreover, the fracture surface of FSPed AZ31 sample exhibits a typical ductile dimple-fracture.