For the better understanding of diffusion process in the buffer material of the HLW disposal system, microstructures of compacted Na-bentonite(Na-montmorillonite content > 98 wt%)were studied by the Scanning Electron Microscopy(SEM) and the X-ray diffractometry(XRD). Consolidated clay particles of 100 μm in width, up to 10 μm in thickness in dry powder of the bentonite became finer ones in compacted bentonite saturated by aqueous solutions. These fine particles, called as stacks, consisted of several or more clay sheets of 100 to 10000 nm in width, 1nm in thickness. The pore structure among stacks(external pore)was controlled by the swelling of stacks, because of the selective hydration of interlamellar space of smectite(internal pore). XRD profiles of the saturated bentonite changed with respect to dry densities of bentonite and salinity of solution. The diffraction peak related to basal spacings, d_<(001)> of 3.52 nm disappeared at high dry densities > 1.0 Mg/m^3. The peak due to the d_<(001)> of 1.88 nm was observed at dry densities of 0.8 to 1.2 Mg/m^3, while that of 1.56 nm was previously reported at dry densities > 1.6 Mg/m^3. The influences of salinity on the d_<(001)> were investigated at a dry density of 0.9 Mg/m^3 and NaCl concentrations of 0.0 M to 0.5 M. There were peaks attributed to the d_<(001)> of 3.52 nm and 1.88 nm. The former peak was observed at the low-saline conditions &lnE; 0.1 M NaCl, while the latter one unchanged with respect to salinity. Absence of d_<(001)> of 3.52 nm is indicative of aggregation of smectite crystal at the high-saline conditions. On the basis of the above observation, the external pore size, L was expressed as a function of the number of clay layers, n per a stack and a dry density ρ_<dry> as follows; [chemical formula] The external pore size abruptly increased with a decrease in the dry density and with an increase in the number of clay layer. Diffusion coefficients of tritiated water(HTO)predicted by the unified molecular dynamics and homogenization analysis for the uniform non-tortuous structure were about twice as large as those experimentally determined. These differences can be seem to be accounted by the tortuosity effects.