The superplastic behavior of a commercial duplex stainless steel has been studied by means of isothermal hot tensile test at temperatures of 850-1050℃ for the initial strain rates ranging from 3×10^-4 s^-1 to 5×10^-2 s^-1. At 960℃, the best superplastic deformation that caused the maximum elongation greater than 840% was obtained for an initial strain rate of 1.2×10^-3 s^-1. At 850℃, the best elongation 500% was achieved for an initial strain rate of 2.5×10^-3 s^-1. During the deformation in higher temperature region,coarse γ grains formed during the prior treatments were broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the δ-ferrite matrix. However, at lower temperatures between 800 and 950℃, the σ phase was formed through the eutectoid decomposition of δ→γ+σ, resulting finally in the stable equiaxed micro-duplex structures with δ/γ and γ/σ, respectively.The precipitation of the σ phase played an important role in improving the superplasticity at 850℃. The strain-rate sensitivity coefficient, m-values, were also determined by the strain rate change tests. The microstructure studies show that the superplastic process occurs mainly by the local work hardening and the subsequent dynamic recrystallization and a grain boundary sliding and grain switching mechanism.