Superplastic deformation behaviors of a relatively coarse-grained AZ80 magnesium alloy sheet have been investigated at temperature ranging from 350 to 430 °C and at the strain rates ranging from 1.31 × 10 -4 to 1.31 × 10 -2 s -1 by uniaxial tensile tests. Superplasticity with the maximum elongation to failure of 239% was obtained at 430 °C and the strain rate of 6.56 × 10 -4 s -1 and its strain rate sensitivity exponent, the value of m is high up to 0.49. Fine interior dynamic recrystallized (DRX) grains distributed along primary grains were observed during superplastic deformation, but primary grains were not found significantly grown up. In addition, micro-cavities and their coalescences were also observed in the superplastic deformation of the relatively coarse-grained AZ80 magnesium alloy sheet. Grain boundary sliding (GBS) was considered to be the main deformation mechanism during the superplastic deformation. Dislocation creep controlled by atom diffusion through interior DRX and primary grains is suggested mainly to accommodate the GBS in superplastic deformation. Copyright © 2011 VBRI press.