Superplasticity and microstructural change during deformation of a 25Cr-7Ni-3Mo α⁄γ duplex stainless steel having a microduplex structure have been studied. The specimen of microduplex structure with fine α and γ grain sizes (∼1 μm) showed high strain rate superplasticity at 1273 K. Elongation over 1700% was obtained at a strain rate of 1.7×10⁻² s⁻¹. Even at 1.7×10⁻¹ s⁻¹ elongation was near 1000%. α⁄α boundaries were low angle ones before the superplastic deformation and were changed to high angle ones by dynamic recrystallization in the early stage of deformation. During deformation up to 1300% elongation after the recrystallization of the α phase, dislocations were scarcely observed in the whole of the specimen. It was concluded that the grain boundary sliding is a predominant deformation mode during superplastic flow in the present steel and the role of dynamic recrystallization is to make the structure suitable for the grain boundary sliding in the early stage of deformation. The mechanism of dynaimic recrystallization was briefly discussed.