The superplastic behavior of a commercial duplex stainless steel has been studied by means of isothermal hot tensile testat temperatures of 850-1050℃ for the initial strain rates ranging from 3×l0-4 s-1 to 5X10-2 s-1. At 960℃, the best superplastic de-formation that caused the maximum elongation greater than 840% was obtained for an initial strain rate of 1.2×10-3 s-1. At 850℃, thebest 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 γparticles within the δ-ferrite matrix. However, at lower temperatures between 800 and 950℃, the σ phase was formed through theeutectoid 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 coeffi-cient, m-values, were also determined by the strain rate change tests. The microstructure studies show that the superplastic processoccurs mainly by the local work hardening and the subsequent dynamic recrystallization and a grain boundary sliding and grain switching mechanism.