Tensile samples of a modified 9Cr-1Mo steel after tempering at 823 and 983 K subsequent to a normalizing treatment at 1373 K (designated as 550T and 710T, respectively) were deformed in tension at temperatures in the range of 77 to 373 K both by constant initial strain-rate tests at ε + ̇ = 1.1 · 10-4/s and strain-rate change tests between 1.1 · 10-4 and 1.1 · 10-3/s at εtot = 0.05, 0.07, 0.1, 0.12, and 0.14. The 710T specimens showed an increase in ductility and a marginal increase in strength with decreasing test temperature, whereas the 550T specimen exhibited an increased flow stress with very low ductility. The thermal component of flow stress (τ*), the activation volume (υ*) and activation energy (ΔH) were strongly affected by the microstructure. A decrease in τ* observed in the 710T specimens, compared with that of pure iron, at temperatures below 200 K can be attributed to solid solution softening by the dissolved nitrogen atoms. Further, the variations in υ* and ΔH with stress and temperature supports the importance of the Peierls mechanism for plastic deformation, and the movement of dislocations by double kink formation was found to be strongly affected by free nitrogen atoms.