Under the TITAN project, in order to determine the contributions of different microstructural features to strength and to deformation mode, microstructure of deformed flat tensile specimens of irradiated reduced activation F82H IEA and its joint were investigated by transmission electron microscopy (TEM), following tensile test and fracture surface examination by scanning electron microscopy (SEM). After irradiation, changes in yield strength, deformation mode, and strain-hardening capacity were seen, with the magnitude of the changes dependent on irradiation temperature. Irradiation to F82H IEA at 573 K 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. While, irradiation at 773 K had little effect on strength, but a reduction in strain rate caused a decrease in ductility. SEM revealed fracture surfaces showing a martensitic mixed quasi-cleavage and ductile-dimple fracture in all samples. TEM have exhibited defect free bands (dislocation channels) in the necked region irradiated at 573 K. This suggests that dislocation channeling would be the dominant deformation mechanism in this steel irradiated at 573 K, resulting in the loss of strain-hardening capacity. While, the necked region of the irradiated F82H IEA joint, where showed less hardening than F82H IEA, has showed deformation bands only. From these results, it is suggested that the pre-irradiation microstructure, especially the dislocation density, could affect the post-irradiation deformation mode.