Microstructure and mechanical properties of friction-stir-welded Al-5.50Mg-0.45Mn and Al-5.50Mg- 0.45Mn-0.25Sc-0.10Zr (wt.%) alloys were investigated by tensile tests and microscopy methods. The results show that the yield strength, ultimate tensile strength and elongation of the Al-Mg-Mn joint are (191±3) MPa, (315±1) MPa and (4.8±1.9)%, respectively. The corresponding values of the Al-Mg-Mn-Sc-Zr joint are (288±5) MPa, (391±2) MPa and (3.4±1.0)%. The Al-Mg-Mn-Sc-Zr joint has smaller grain size, lower average misorientation angle and higher low-angle grain boundary fraction than the Al-Mg-Mn joint. Both the two joints fracture at the weld nugget zone (WNZ), and Al₃(Sc_1-xZr_x) nano-particles with a mean size of (9.92±2.69) nm still remain in this “weakest zone”. Theoretical calculation indicates that nano-particles can provide Orowan and grain boundary strengthening in WNZ, increasing the yield strength of the Al-Mg-Mn joint by 97 MPa.