Fracture toughness is one of the most important material properties that influences the integrity of reactor pressure vessels (RPVs). Methods for reconstitution of fracture toughness specimen have been developed preliminarily in order to increase the number of fracture toughness data of irradiated RPV steels. The specimen adopted in this study was a 12.5mm thick compact specimen (1/2TCT). The size of insert material, which is removed from the assumed broken specimen, is 13 × 15 × 12.5mm3. Materials used were two kinds of unirradiated RPV steels. Welding procedures investigated were CO2 laser beam welding(LBW) and electron beam welding(EBW). Several welding parameters, such as welding current, speed of travel, etc. were varied parametrically. Then optimum welding conditions of both weldings for reconstitution of CT specimen were investigated. It was found that fracture toughness values obtained from the reconstituted CT specimens of two steels are almost the same as those from original specimens in both the transition and the upper shelf temperature ranges. It turns out that the above methods can be applied for the reconstitution of irradiated fracture toughness specimen in hot cell.