Abstract: Most of the existing CT technologies used for study of the mechanical properties of concrete at mesoscopic level are limited to a stage of qualitative observation and statistical analysis, lacking the capability of quantitative analysis based on mathematical theory. This article presents an application of the set theory to damage-fracture evolution of concrete specimens in CT test, and develops a new quantitative method for analysis of damage process using CT test data. By the damage-fracture evolution theory and the definition of λ1~λ2 intercepted sections, the CT image is divided into a crack area (P0~0.35), a hardened cement paste area (P0.35~0.7), and an aggregate area (P0.7~1), so that different components of concrete in each subarea can be examined quantitatively. Then, the quantitative variations of P0~0.35, P0.35~0.7 and P0.7~1 can be calculated with the change of CT test loading for the whole image region or any interested sub-regions. The results show that this new method is successful in quantitative describing the strain localization of specimens in failure process and provides an effective tool for study of concrete failure.