Abstract: Fracture toughness of S08-3 steel welded joints was investigated for large nuclear vessel. The results show that the fracture toughness was different for base metal,weld metal and heat affected zone with larger sample size. The weld metal had superior fracture toughness compared to heat affected zone,and the fracture toughness of base metal was lowest. Sample size directly affected the stress state at the crack Lip which endured plane stress in small sample. The base metal,weld metal and heat affected zone had good toughness with dimple fracture morphology. However, the crack tip in large sample endured plane strain condition. The fracture morphology of base metal presented quasi-cleavage and cleavage mixed mode. The heat af-fected zone was typical quasi-cleavage mode, and the weld metal revealed dimple fracture.