Abstract: The hot deformation behavior of 9Cr reduced activation martensitic steel was investigated by compression testing on a Gleeble-1500D simulator in the temperature range of 950 to 1200℃ and the strain rate range of 10^-2 to 10 s^-1, and the microstructural evolution was analyzed by metallography. Based on measured data in the strain range of 0.15 to 0.8, material parameters in the Arrhenius-type hyperbolic-sine equation were solved by using regression analysis methods and then used to fit the deformation. A constitutive equation which relates the flow stress to temperature, strain rate and strain was established by mathematical methods. The values of peak stress, critical stress, peak strain, critical strain and grain size for dynamic recrystallization (DRX) were determined from the true strain-true stress curves and their equations related to the Zener-Hollomon parameter were obtained.