Abstract: In this paper, from the linear representation of linear attenuation coefficient of mixture, density variations and CT number variations are induced by microcracks under stress, the formula between relative density variations and relative CT number variations is obtained without impurity. With further analyzing, a formula between relative volume percent of crack defect variations and relative CT number variations is obtained respectively, without impurity and with impurity, and we extend conclusion of impurity to impurities. All these lay a theoretical foundation for material fatigue life prediction by using CT.