Abstract: The bentonite pellet mixture has been proposed as a candidate backfilling material for technological gaps in deep geological repositories for disposal of high-level radioactive waste. Its thermal conductivity plays an important role in the safety assessment of the repository. In this study, a series of thermal conductivity tests are conducted on the GMZ bentonite pellet mixtures with different grain-size distributions. The results demonstrate that as the pellet size increases, the thermal conductivity of the mixtures firstly increases and then decreases for the specimens freely filled. However, the thermal conductivity monotonically decreases with the increase of the pellet size for the specimens packed at a given dry density. During the hydration process, the time-history curves of thermal conductivity for the pellet mixtures with different grain-size distributions can be approximately divided into a rapidly increasing stage and a stable stage. Meanwhile, the evolution and the stable time of thermal conductivity are both influenced by the particle composition. The bentonite pellets are swelled to fill the inter-pellet pores upon hydration, and the infiltration frontier gradually moves from the top/bottom part to the middle one, which is accompanied by a gradually structural transformation from a granular structure into a continuously homogeneous one. After approaching to the saturated state, the relationship between the thermal conductivity and the dry density for the bentonite pellet mixtures shows a good consistency to that of the compacted bentonite blocks.