A reduced activation ferritic/martensitic steel, F82H (IEA heat), developed for fusion energy applications was irradiated at 300 and 500°C to 5 dpa in the High Flux Isotope Reactor (HFIR). Changes in yield strength, deformation mode, and strain-hardening capacity were seen, with the magnitude of the changes dependent on irradiation temperature. Irradiation at 300°C led to a significant loss of strain-hardening capacity with a large change in yield strength. There was a tendency for a reduction in strain rate to cause a decrease in yield strength and elongation. Irradiation at 500°C had little effect on strength, but a reduction in strain rate caused a decrease in ductility. In order to determine the contributions of different microstructural features to strength and to deformation mode, transmission electron microscopy (TEM) specimens were prepared from the gage sections of the tested (strained) flat tensile specimens and examined; fracture surfaces were examined by scanning electron microscopy (SEM). The fracture surfaces showed a martensitic mixed quasi-cleavage and ductile-dimple fracture in the center at both irradiation temperatures. The microstructure in the necked region irradiated at 300°C showed defect free bands, which may be dislocation channels. This suggests that dislocation channeling could be the dominant deformation mechanism in martensitic steels irradiated at 300°C, resulting in the loss of strain-hardening capacity.