The superplastic response of commercial 5083 alloy( Al-4. 42Mg) under uniaxial tension at strain rates ranging from 5 × 10- 5to 10- 2s- 1in the temperature interval 400- 550 ℃ was systematically studied in this paper. The tension test was conducted on samples of rolling direction. The maximum elongation-to-failure of 486% was found at 500 ℃ and strain rate of 10- 4s- 1. To identify the main characteristics of superplastic deformation and to determine the mechanism of superplastic deformation of the alloy,the microstructure and fracture of the alloy were analyzed as a function of strain,strain rate and temperature using optical microscopy( OM) and scanning electron microscopy( SEM),the apparent strain rate sensitivity exponent m a and the apparent activation energy Q a were also studied. Based on the fracture analysis and the calculated data of m a and Q a,it is suggested that the dominant deformation mechanism in the present alloy is grain boundary sliding( GBS) during the best deformation condition. The superplastic response of commercial 5083 alloy (Al-4. 42Mg) under uniaxial tension at strain rates ranging from 5 × 10- 5to 10- 2s- 1in the temperature interval 400-550 ° C was systematically studied in this paper. The tension test was The maximum elongation-to-failure of 486% was found at 500 ° C and strain rate of 10- 4s- 1. To identify the main characteristics of superplastic deformation and to determine the mechanism of superplastic deformation of the alloy, the microstructure and fracture of the alloy were analyzed as a function of strain, strain rate and temperature using optical microscopy (OM) and scanning electron microscopy (SEM), the apparent strain rate sensitivity exponent ma and the apparent activation energy Q a were based on the fracture analysis and the calculated data of ma and Q a, it is suggested that the dominant deformation mechanism in the present alloy is grain boundary sliding (GBS) during the best de formation condition.