Degradation of fracture strain and fracture toughness due to neutron irradiation in austenitic stainless steels are important issues to evaluate for plant life extension of light water reactors (LWRs). However, few efforts have been made to develop a prediction method for fracture strain and toughness. In this paper, ductile fracture of irradiated material was studied based on the irradiation-induced defects forming process combined with a ductile fracture process model i.e., void growth and coalescence model, and a model for fracture strain was developed as a function of fluence, applied stress and work hardening rate. Tensile tests of austenitic stainless steels irradiated up to about 1026 n/m2 in an LWR were conducted at room temperature and 561 K. Fracture strain decreased with the increase of neutron fluence, but no specimens showed any brittle features. The proposed model for fracture strain had good correlation with experimental results. The fracture surfaces were investigated by scanning electron microscope (SEM). Dimple diameter decreased with the increase of neutron fluence and this fact was presumed to support the propriety of the proposed fracture strain model. Fluence dependence of fracture toughness was studied based on the proposed fracture strain model. The proposed fracture toughness model agreed well with the open literature. Fracture toughness JIC of base metal and weld metal of Types 304 and 316 stainless steels neutron irradiated and tested at 673–700 K were predicted by the fracture toughness model. JIC of base metal was higher than the weld metal in the whole fluence range. JIC of base metal and weld metal decreased above the fluences of about 5×1025 n/m2 and 1×1025 n/m2 (E>0.1 MeV), respectively. Consequently, the proposed ductile fracture strain and fracture toughness models were applicable to predict those values for base metal and weld metal of neutron irradiated austenitic stainless steels.