Diffusion behavior of water through a composite filled with plate-like particles was numerically analyzed using the finite element method to examine the excellent water barrier property of nylon-6/clay hybrid (NCH). In NCH, the clay was dispersed as 100%∼100%∼1 nm plates. The calculated results were compared with experimental ones with regard to relative diffusivity, which is defined as the ratio of the diffusion coefficient of NCH to that of nylon 6. This comparison gave good agreement between them. The computer simulation demonstrated that the water barrier property of NCH depended on the volume fraction, size, orientation, and placement of the clay; and that it is mainly increased by the obstacle effect of highly oriented clayey plates.