This paper proposes a method to predict the thermal ageing effect on fracture toughness of CF8M cast stainless steel. The proposed method is based on multi‐axial fracture strain combined with finite element damage analysis to simulate ductile tearing. Multi‐axial fracture strain loci of un‐aged and aged CF8M are determined by analyzing notched bar tensile test. It is shown that the thermal ageing effect on multi‐axial fracture strain loci can be characterized by one constant. It is further shown that J‐resistance curves of un‐aged and aged CF8M can be predicted well from finite element damage analysis using multi‐axial fracture strain loci. Implication of present results to practical application of crack assessment of aged cast stainless steels is discussed.