Abstract: Based on the dislocation reaction theory and the Avrami equation,a two-stage constitutive equation model is developed to describe dynamic recovery and dynamic re-crystallization,which couple the effect of the dynamic strain aging in the alloy during hot deformation. With the Gleeble 3500 simulation testing machine,uniaxial thermocompression tests to the P91 heat resistant steel are carried out under the conditions of 950～1 200 ℃ and 0.01～10 s^-1,which results in the true stress-true strain curves thereof. In combination with the experimental data concerning the flow stress of the P91 steel,the material parameters of the said constitutive equation are determined by using the non-linear dada-fitting method. The calculation result from the constitutive equation shows that the calculated curves are in compliance with the experimental curves,which means that the constitutive equation of the P91 steel is capable of predicting the flow stresses of the steel being deformed at the hot working conditions of temperature ranging from 950 ℃ to 1 200 ℃,and strain rating from 0.01 s^-1 to 10 s^-1.