Abstract: Taking the reactor pressure vessel as an example, the theoretical relationship of fatigue strength reduction factor (K_f) and stress concentration factor( K_t ), the reasonable values of K_f and K_t, the principles of equivalency of K_f and K_t in engineering project and the methods and techniques of how to use K_f and K_t in fatigue analysis are presented. An example of fatigue analysis for screw threads of reactor pressure vessel (RPV) stud with certain K_f is given. Consequently, definitions and differences of K_f and K_t are classified. Opinions on the application of K_f to high strength alloy stud are given, which can be used as a reference to the stud selection and the structure design in the engineering application.