High-density bentonite pellets have been considered as an alternative sealing or buffer material for nuclear waste repository. Investigation on the packing behaviour of pellets is of great interest due to the significant impacts of global density on the hydro-mechanical behaviors of the material. In this manuscript, the packing behaviour of mono-size, binary-size, ternary-size and multi-size mixtures of crushed bentonite pellets were experimentally investigated. The effects of particle size ratio and distribution on packing dry density were analyzed. Results indicate that, for mono-size mixtures, the packing dry density can be decreased by the agglomerating effect of fine particles and wall effect of the container. For the binary-size, ternary-size and multi-size mixtures, there is an optimal particle size distribution corresponding to the peak packing dry density, which was found to be closely related to the particle size ratio of the finest size class to the coarsest one. The effects of particle size ratio and distribution can be interpreted by the ‘net positive effect’ representing the competition between the positive effects (filling and occupying effects) and the negative effects (loosening, wall, wedging and agglomeration effects) of particle interactions.