Abstract: By using the thermecmastor-Z thermomechanical simulator,hot compression test was performed.The static and dynamic CCT curves of 20MnSi and 20MnSiNb steel were built,and the effects of cooling rate,addition of Nb element and deformation on γ→α continuous cooling transformation of Nb microalloyed steel were studied by mean of thermal dilatation technology and microstructure observation.The results showed that the ferrite transformation temperature is lowered and the grain size is refined with the increasing of cooling rate;the γ→α transformation is accelerated and the ferritic grain was refined further after deformed due to the precipitation of Nb(C,N);and the deformation can also promote the ferritic transformation.