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 y grains formed during the prior treatments were broken into spherical particles, resulting in a homogeneous dispersion of y particles within the ?-ferrite matrix. However, at lower temperatures between 800 and 950℃, the o phase was formed through the eutectoid decomposition of ?-?+?, resulting finally in the stable equiaxed micro-duplex structures with δ/γ and γ/σ, respectively. The precipitation of the o phase played an important role in improving the superplasticity at 850℃. 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 ranges ranging from 3 x 10-4 s-1 to 5 x 10-2 s-1 At 960 ° C, 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 ° C, the best elongation was 500% achieved for an initial strain rate of 2.5 × 10-3 s-1. During the deformation in higher temperature region, coarse y grains formed during the prior treatments were broken into spherical particles, resulting in a homogeneous dispersion of y particles within the? -ferrite matrix. However, at the lower temperatures between 800 and 950 ° C, the o phase was formed through the eutectoid decomposition of? -? + ?, resulting finally in the stable equiaxed micro-duplex structures with δ / γ and γ / σ, respectively. The precipitation of the o phase played an important role in improving the super plasticity at 850 ℃.