The microstructure with uniform equiaxed fine gain was obtained by the thermo-mechanical processing(TMP)for the 5083 Al alloy(Al-4.56%Mg-0.61%Mn)plate.Uniaxial tensile test was carried out at a temperature range of 500-570℃and a strain rate range of 4.17×10 ~(-4 )s ~(-1)-1×10 ~(-2)s~(-1) .Maximum tensile elongation 530%was obtained at 550℃and strain rate 4 ε&=4.17×10~(-4)s~(-1) .Dislocations were observed in grain interiors and at grain boundaries during uniaxial tensile deformation by transmission electronic microscopy(TEM),respectively.Results show that the grain boundary sliding(GBS)accommodated by dislocation motion is the principal reason for superplastic deformation.The cavities and fracture were observed during uniaxial tensile deformation of the alloy by scanning electronic microscopy(SEM),indicating that linkage of cavities in large re-gion would induce failure of the material.Moreover,presence of liquid phase at grain boundary also affects the superplastic deformation and behavior of cavities.