The effect of irradiation on cleavage fracture toughness in the ductile-brittle transition range of RPV steels was studied using precracked Charpy-V (PCCv) specimens. Four kinds of RPV steels of ASTM A533B class 1 made by Japanese steel makers were used for this study. Neutron irradiation to PCCv specimens was carried out at JMTR. An approach based on the weakest link theory was applied to analyze the specimen size effect and the scatter of fracture toughness. The scatter of fracture toughness was confirmed to have a Weibull type distribution. Irradiation effect on the scatter was found insignificant. The fracture toughness transition curves were determined by PCCv data according to the master curve approach and the Japanese code. The shift of the master curve due to neutron irradiation was somewhat greater than the Charpy 41J shift. The scheme to estimate the lower bound of fracture toughness was also studied.