Abstract: In-diffusion coupled with the abrasive peeling technique was employed in the study of diffusion behavior of the strong sorption radionuclide ²³⁷Np(Ⅴ) diffusion on Opalinus Clay. The stainless steel filter disc was used to confine the sample. It is very necessary to investigate the influence of the filter effect on the modeling. The diffusion parameters, such as effective diffusion coefficient D_e, distribution coefficient K_d and rock capacity factor α, were obtained by an in-house computer code to model the in-diffusion experiment results. Filter effect was considered in the computer code by introducing the effective diffusion coefficient D_fe of ²³⁷Np(Ⅴ) in the filter. The results show that it is necessary to consider the filter effect. Moreover, Stokes-Einstein question can be used to design and optimization of in-diffusion experiments in which no experimental data D_fe are available.