Abstract: Bentonite, as buffer materials in disposal repository, will be under long term effect of decay heat which released from nuclear waste sealed in the canister. Whether the expansion self-healing ability of bentonite can be maintained after sustained high temperature has not been clearly concluded. The powdery bentonite was heated in muffle furnace at a constant temperature (105 ℃). But the heating periods were different, and the longest one was 90 d. A series of tests was carried out on the heated samples. The results show that the free swelling ratio and swelling pressure decrease dramatically with heated time. X-ray diffraction spectrums of montmorillonite demonstrate that the adjacent lattice distances is shortened, silica oxide cementation occurs after continuous heating for 90 d, which results in aggregation of montmorillonite particles. It can be inferred from infrared spectrum and thermogravimetric testing results that the hydration of interlayer ions of montmorillonite is removed by continuous heating, and some Na and Mg ions escape with the evaporation of water. The combination of the two causes the condensation of montmorillonite particles. This phenomenon is also found by SEM that montmorillonite particles closely linked to each other instead of distinctly stacked as original state. Moreover, this condensation or cluster behaviors are irreversible, which could be confirmed from particle size distribution curves of heated bentonite. Because the clusters of bentonite (heated 90 d) were not dispersed obviously, even after soaking 28 d.