Abstract: 3-mm-thick AZ31B magnesium alloy plates were successfully joined by conventional friction stir welding and cold source-assisted friction stir welding. The effect of microstructure on mechanical properties of the welds were investigated by electron backscatter diffraction, transmission electron microscopy, and tensile tests. The obtained results showed that the welding peak temperature was significantly reduced, and the cooling rate after welding also remarkably increased due to liquid CO₂ cooling. The decreased welding peak temperature created favorable condition for the activation of 10-12 twinning behavior. The 10-12 twins reduced the basal texture intensity, and further refined the grain size. Because of the enhanced cooling rate, massive dislocations which generated during the welding process were maintained in the grain interior. Therefore, the weld obtained by cold source-assisted friction stir welding exhibited a fine-grained structure with massive 10-12 twins and dislocations. During the tensile tests, the main strengthening mechanisms were attributed to grain boundary strengthening and dislocation strengthening. The twin boundaries can effectively coordinate strain and reduce stress concentration by absorbing and decomposing dislocations which produced in plastic deformation. Therefore, the weld showed reasonable strain hardening behavior and a good matching of strength and ductility.