This paper suggests a method to predict thermal ageing effect of CF8A material under cyclic loading effect. To achieve the goal, FE damage model based on multi-axial ductility theory is used. The model describes the local failure by fracture strain energy and stress-triaxiality. The fracture strain energy of aged CF8A can be determined from tensile test and that of aged CF8A can be assessed adopting thermal ageing constant "C". To simulate the material behavior under cyclic condition, two assumptions are made. One is about energy conservation and the other is about crack opening. From the assumptions, same fracture criteria are used to predict fracture toughness under cyclic loading condition.