This paper focuses on the constitutive behavior of the F82H ferritic/martensitic steels internationally investigated in the frame of the fusion material development program. Tensile tests were carried out in the temperature range (80-723 K) at different strain rates. A decomposition of the how stress into a yield stress plus a hardening stress component is proposed. The temperature, strain and strain rate dependence of the stress is discussed based on the current available data. The strain-hardening is calculated and two strain-hardening stages are identified along the deformation curves between the yield stress and the necking instability. Finally, semi-empirical constitutive relations, which are partly based on dislocation mechanics, are derived. (C) 2001 Elsevier Science B.V. All rights reserved.