Buffer materials filled in the nuclear waste deep geology disposal should not only own to excellent capacity of isolation and adsorption, but also process high thermal conductivity. Hence, choosing natural graphite powders as additives can improve the thermal conductivity of Na-bentonite. These mixtures have both isolated and heat-conducted properties. The graphite powders, whose maximum grain sizes are 50, 100, 200 and 325 mesh, respectively, are added into the bentonite at fixed graphite adding ratio(20%,Wt.). The effect of graphite particle size on the hydro-mechanical behaviors of graphite-bentonite mixtures is studied by conducting some experiments including free swelling ratio, constant volume swelling pressure and permeability. It can be found that the maximum particle size 100 to 200 mesh of graphite would be the best choice to obtain the lowest permeability and highest swelling pressure for buffer material at the same graphite content. It can be ascribed to the contacted mode between graphite and bentonite. The graphite particles are flat, especially for the large size ones, and are easier to form macro-pores at the edge of them, even if after being compacted. As for smaller size graphite particles, there are existing larger contacted areas between graphite and bentonite. Plus the hydrophobicity of graphite, more leakage passage would be formed at the graphite-bentonite interfaces. These conclusions provide some scientific references for preparing buffer material used in the nuclear waste deep geology disposal.