The influence of thermal aging on fracture properties, transition temperature of V-notch Charpy impact test, elastic-plastic fracture toughness, fatigue crack growth behavior and stress corrosion cracking of the base metal and welded joint of A533B steel were examined. The chemical compositions of the base metal and weld wire were adjusted, referring to those of reactor pressure vessels of old Japanese commercial nuclear power plants constructed before 1970. A step-cooling heat treatment was used as embrittling treatment. The transition temperature of the base metal and welded joint subjected to step-cooling rises approximately 30°C due to the segregation of phosphorus. However, the step-cooling heat treatment has little influence on the fracture toughness, the crack growth behavior and stress corrosion cracking.