Abstract: The hot deformation behavior of FV520B martensitic stainless steel was investigated by single pass isothermal compression test on the Gleeble-3500 thermal simulation machine at the deformation temperatures of 850-1 150℃ and stain rates of 0.005-5.000 s^-1. According to the stress-strain curves and on the basis of Zener-Hollomon parameter and the Arrhenius hyperbolic-sine equation, the constitutive equation of the stainless steel during high temperature compression was established, and was corrected and verified by experiments. The results show that the flow stress of FV520B martensitic stainless steel decreased with the increase of deformation temperature or the decrease of strain rate. The dynamic recrystallization occurred obviously in the stainless steel at 0.005 s^-1, 1 000-1 150℃ and 0.050-5.000 s^-1, 1 075-1 150℃. The errors between the predicted flow stresses by the constitutive equation and the experimental values were relatively large under the conditions of 0.005 s^-1, 850℃ and 5.000 s^-1, 850℃ or 925℃. After the constitutive equation was corrected, the correlation coefficient between the predicted flow stress and the experimental value was 0.997 88, and the average relative error was 2.225%. The corrected constitutive equation predicted the hot deformation flow stress of the stainless steel accurately.