Abstract: Hot compressive tests were carried out on a Gleeble-1500 thermal simulation test machine to investigate the hot deformation behavior of T91 steel at elevated temperatures of 1 100 to 1250℃ and strain rates of 0. 01 to 1 s^-1. The relations of flow stress to strain rate and deformation temperature were analyzed for calculating the stress exponent and the deformation activation energy at the elevated temperatures. The constitutive equation was constructed by introducing the Zener-Hollomon parameter. The dynamic recrystallization map and the processing map of plastic deformation of T91 steel at the elevated temperatures were drawn. The true stress-strain curves of the steel show a bimodal characteristic. Distinct dynamic recrystaflization takes place in the steel, and its mechanism is continuous dynamic recrystallization. The deformation activation energy of the steel is 484 kJ·mol^-1. The instability zones and optimum processing parameters of hot deformation under the condition can be attained by using the two maps. The steel has excellent mechanical properties when rolled at deformation temperatures of 1 200 to 1 250℃ and strain rates which are not greater than 0. 1 s^-1.