Radiation hardening/embrittlement was investigated for clad-heat affected zone (clad-HAZ)) of reactor pressure vessel (RPV) of Hamaoka unit-1 boiling water reactor (BWR) under decommissioning process. The neutron irradiation fluence in the HAZ of the RPV steel was 1.5 × 1022 n/m2 (E > 1MeV). The shift in ductile-brittle transition temperature, ΔDBTT, was measured to be +26 °C using 1/3-sized Charpy specimens, while the increase in Vickers hardness, ΔHv, was 19 HV in the clad-HAZ of 1.6 mm depth from the clad boundary. Atom probe tomography (APT) was conducted at the locations 0.1, 0.8, 1.6. 4 and 10 mm away from the clad boundary, and revealed that solute clusters were formed in both clad-HAZ and base metal (10 mm depth), and the clusters were rich in Mn, Ni, Si, Cu with the averaged diameter of about 2.5 nm and the number density of 2 × 1022/m3, being almost independent of depth location. As for the chemical compositions of the clusters, Mn content increased with decreasing Cu content. This suggests that there might be site completion between Mn and Cu for clustering. Radiation hardening of clad-HAZ induced by the formation of solute clusters was estimated on the bases of the dispersed barrier hardening (DBH) model in terms of , and well interpreted with α = 0.06–0.08. The ΔDBTT- plot, of which the data was measured with 1/3 size specimens sampled from the clad-HAZ at 1.6 mm depth, agreed with the previous surveillance test results measured in surveillance tests with standard size specimens of base metal.