The relation between resistivity and mechanical properties, such as hardness and the ductile-brittle transition temperature (DBTT), in heat-affected zone (HAZ) of welded A533B pressure vessel steel (PVS) has been investigated by means of a small specimen test technique before and after neutron irradiation. A submerged arc welded thick plate of A533B for plant use was supplied by Japan Steel Works, Ltd. Neutron irradiation was performed in the Japanese Materials Test Reactor (JMTR) at 290°C up to 1.3 × 1024 n/m2. Resistivity measurements with a pair of potential probes, 1 mm distance apart, were performed on the surface of specimens at different locations across the fusion boundary. Before the irradiation, the resistivity of the weld was higher than that of the base metal and the resistivity in the HAZ gradually reduced with increasing distance from the fusion boundary. After the neutron irradiation, the resistivities at all the locations were reduced. Post-irradiation annealing experiments revealed that resistivities of base metal and HAZ almost recovered after annealing to 500°C, while only a part of the recovery was observed for the resistivity of the weld. On the contrary, the recovery of irradiation hardening was faster in the weld than in the base metal. It is considered that the reduction of resistivity is due to precipitation of solute elements, such as manganese, nickel and copper. The correlation between resistivity change and irradiation hardening is also discussed.