The 2 mm-thick α-brass plates were successfully joined using conventional friction stir welding (CFSW) with air cooling and rapid cooling friction stir welding (RCFSW) with liquid CO₂ cooling. The microstructure and mechanical properties of the two welds were carefully investigated by electron back-scattered diffraction and transmission electron microscopy. The stir zone of CFSW exhibited homogeneous equiaxed grains, while the stir zone of RCFSW showed a heterogeneous grain structure, i.e. ultrafine grains containing massive dislocations and nano twins. Compared with the CFSW, yield strength and ultimate tensile strength of RCFSW were increased by 31% and 24%, respectively. The enhanced yield strength and improved strain hardening capacity were attributed to grain boundary strengthening and dislocation strengthening. Furthermore, good ductility was achieved due to the released stress concentration of the nano twins caused by the plastic deformation.