Bentonite is attracting greater attention as buffer and backfill materials for repositories of high-level nuclear waste because these materials must have high swelling property and low permeability. To contribute the design of buffer and backfill materials, this study investigated the swelling properties of sand-bentonite mixtures, of which bentonite contents are within 5-100%, and four typical kinds of bentonite produced in Japan and the USA. The experimental works clarified the influence of bentonite-content, dry density, montmorillonite-content of bentonite and excangeable-cation-compositions upon the swelling characteristics of materials. Moreover, it was found that the authors' parameter “Swelling volumetric strain of montmorillonite” can easily evaluate the maximum swelling pressure and the maximum swelling strain of sand-bentonite mixtures and various kinds of bentonite at various dry densities and bentonite contents.