Abstract: The change of phase proportion of the austenite (γ) and ferrite (δ) in the microstructure of 2205 duplex stainless steel during thermal deformation was investigated by single-pass reduction simulation. The results showed that there are not only the phase transition between austenite and ferrite but also a dynamic softening mechanism due to the transition during thermal deformation of the stainless steel. The influence of deformation rate on phase transition during thermal deformation was found in such a way that the δ&rarrγ transition is dominant if the deformation rate is very small, but the portion of γ&rarrδ transition increases with increasing deformation rate.