SA508 cl.3 reactor pressure vessel steel plates were overlay cladded with type ER309L welding consumables by submerged arc welding. The microstructure of the clad was composed of fcc austenite, a few percent of bcc delta-ferrite and brittle a phase. The irradiation-induced hardening and its related mechanical property changes of the clad were investigated using tensile, small punch (SP) and electrical resistivity measurements. Neutron fluences of irradiated specimens at 290 C were ranged from similar to5 x 10(18) to similar to5 x 10(19) n/cm(2) (>1MeV). Not only yield stress but electrical resistivity were increased with irradiation and the delta-ferrite showed an approximately 1.5 times higher hardening than the austenite matrix. The general hardening may come from the irradiation-produced fine size defect clusters in the austenite matrix. The SP ductile/brittle transition temperature (SP-DBTT) moved to higher temperature with irradiation, and the SP-DBTT was substantially influenced by the amount of brittle a phase.