The microstructure changes of modified sodium bentonite from Gaomiaozi after gamma rays irradiation-thermal sequential aging were studied using X rays diffractometer. The results showed that the microstructure of modified sodium bentonite with different cumulative doses had little change under the experimental conditions in this study, and the same results were obtained in the case of modified sodium bentonite after aging the same time at different temperatures. The results also indicated that the cumulative dose and the aging temperature had no significant effects on the microstructure change of modified sodium bentonite. The water absorption experiment showed that the changes of the position of the diffraction peak of modified sodium bentonite with aging time were mostly dependent on the changes of the number of crystal water of montmorillonite with aging time. Furthermore, it was found that the changes were reversible. The modified sodium bentonite from Gaomiaozi could maintain its ability of water absorption unchangeable after 5 000 kGy γ-rays irradiation and thermal aging for 8760 h at 230℃.Cited