Low-cycle fatigue results at ambient temperature under diametral strain-controlled conditions of the reduced activation ferritic/martensitic steel, F82H IEA heat before and after neutron irradiated samples are reported. The results show that cyclic softening behavior is the main mechanical feature observed in this material. A detailed analysis for the hysteresis loops was carried out to determine the friction and back stresses. The friction stress is equivalent to the resistance which the dislocations have to overcome to keep moving in the lattice. The back stress depends on the density of long-range impenetrable obstacles that are created by the dislocations movement such as pile-ups. The cyclic softening of F82H IEA heat is related with the decrease of the friction stress. Moreover, the friction and back stress behavior of neutron irradiated samples show significantly different behavior than those of unirradiated samples. (C) 2008 Elsevier B.V. All rights reserved.