In order to investigate mechanisms in recovery of irradiation-induced hardness by thermal annealing for pressure vessel steels containing copper, Fe-0.6 wt.% Cu model alloys irradiated to a dose of 0.0055 dpa at 290°C in JMTR were isochronally annealed at temperatures ranging from 250 to 600°C (ΔT = 50°C, Δt =15 min). Vickers hardness measurements showed that 70% of the irradiation-induced hardness was eliminated by the annealing. TEM examinations revealed that three kinds of precipitates were formed in the specimens; those are small copper precipitates (disk-shaped, 3–8 nm in diameter), ordered structures of possibly Cu3Fe (L12-type, a0 = ~0.343 nm) and Fe3O4 particles (structure of spinel-type). Copper precipitates slightly increased the number density and size with the post-irradiation annealing below 400 °C, indicating clearly that copper precipitates contributed to hardening, though the degree may be small. The ordered structure was proven by the occurrence of superlattice reflections in selected area diffraction patterns. The post-irradiation annealing reduced the intensities of the superlattice reflections and the zone of Cu3Fe-structure. The ordered structure of Cu3Fe was observed to be destroyed by thermal annealing, resulting in a remarkable decrease in hardness. The hardness during thermal annealing was concluded to be governed by two competitive processes of hardening by the growth of copper precipitates and softening by the order-disorder transformation of Cu3Fe.