In this study, diffusion behavior of cesium(Cs) with a concentration of 0.1 mM in bentonite (MX80) was investigated using through-diffusion methods with various lengths (0.5, 0.75, 2.0, 2.5 cm), respectively. Additionally, a non-reactive tracer (HTO) was conducted to characterize the physical process in compacted MX80 before cesium. Figure 1 shows that the diffusion process of Cs in synthetic seawater (SW) and groundwater (GW) reached equilibrium after 250 and 500 days, respectively. It addition, it also shows results that indicate that the distribution coefficients (Kd) of Cs in GW are higher than those in SW, which is in agreement with results obtained from the batch method. In fact, it clarifies further that the major sorption mechanism of Cs is ion exchange and that ionic strength (I) is the major factor affecting Cs retardation. The retardation factors (Rfs) of Cs were valued in a 1-D diffusion hypothesis, and Rfs of higher length and diameter (L/D) ratios appeared to be smaller values than those found in smaller L/Ds. This discrepancy might be explained by the difference in the soil/liquid ratio, and it can be taken as another parameter influencing different Kd values. Considering the dependency of the soil/liquid ratio on the Kd, a higher solid/liquid ratio applied in the experiments provided more realistic Kd values in a real geological environment.