Natural Na-bentonite, from Gaomiaozi (GMZ), Inner Mongolia, was tested to study the long-term stability of buffer/backfilling material for high-level radioactive waste geological disposal, . This paper analyzed the effects of γ-rays irradiation on the strength and deformation characteristics of densely compacted Na-bentonite sample. The bentonite was irradiated by cobalt source at different cumulative irradiation doses in laboratory. The mineral composition changes into some more stable silicate and aluminosilicate minerals after irradiation. Then the triaxial shear test and swelling test show that the stability of the microstructure of montmorillonite decreases, and the shear strength and the peak shear strength of bentonite increase with the irradiation dosage under the same confining pressure, due to the change in the mineral composition. With the increase of confining pressure, the strength trend increases gradually slowly, and the effect of irradiation dose on the strength turns to be weaken. It indicates that the confining pressure inhibits the strength increase of bentonite sample. Due to reduction of montmorillonite after irradiation, the swelling deformation property is weakened. The performance is the decrease of the expansion force, free swelling strain, load swelling strain and other indicators.