Abstract: 2 mm thick HSn70-1 tin brass plates were joined using cold source assistant friction stir welding. Defect free welded joint was obtained. The microstructure and its effect on the strengthening mechanism and strain hardening behavior were studied by optical microscopy, electron backscatter diffraction, transmission electron microscopy, and tensile test. The obtained results showed that the rapid cooling effect of dry ice and ethanol mixture eliminated the heat-affected zone, and it also restrained dislocation recovery and grain growth in the stir zone. The stir zone exhibited ultra-fine grain structure with high dislocation density. The ultimate tensile strength of the stir zone was 486MPa, which was 53.8% higher than that of the base material, and the fracture elongation reached to 30%. The yield strength of the stir zone also increased by 47.1% compared to that of the base material, and the yield strength of the stir zone was dominated by solid solution strengthening and grain boundary strengthening mechanisms, while the strain hardening behavior was mainly affected by grain size and dislocation density. The nano-scale deformation twins which generated during the tensile test effectively coordinated plastic deformation and alleviated stress concentration, so the strength of the stir zone was improved without remarkable ductility loss.