Abstract: Bentonite pellet mixture has been proposed as a candidate material for backfilling technological gaps in deep geological repositories for disposal of high-level radioactive waste. Thermal conductivity of this material plays an important role in the safety assessment of the repository. In this study, a series of thermal conductivity tests were conducted on GMZ bentonite pellet mixtures with different gradations. Results demonstrated that as the pellet size increased, thermal conductivity of the mixtures firstly increased and then decreased for the specimens freely filled. However, the thermal conductivity monotonically decreased with increases of the pellet size for specimens packed under given dry density. During hydration, the evolution curves of thermal conductivity with time (λ~t) for the pellet mixtures with different gradations could be approximately divided into a rapidly increasing stage and a stable stage. Meanwhile, the evolution curve and stable time of thermal conductivity were both influenced by the particle compositions. Bentonite pellets swelled to the inter-pellet pores upon hydration and the infiltration frontier gradually moved from the top/bottom part to the middle part, accompanied by a gradual transformation of the specimen from a granular structure into a continuously homogeneous one. Once approaching to the saturated state, the relationship between thermal conductivity and dry density for the bentonite pellet mixtures showed a good consistency to that of the compacted bentonite blocks.