Molecular-dynamics (MD) simulation of the diffusion of water in amorphous polyethylene has been performed under a constant particle number, constant pressure, and constant temperature (NPT) conditions. The three simulation systems with the different water concentration from 0.120% (g/g) to 0.324% (g/g) were tested. A time duration beyond 10 ns at 298 K and at 0.1 MPa revealed that the system retained the amorphous structure and the water molecules came together forming water clusters of various sizes. The amount of water clustering depended on the water concentration and we observed irreversible clustering of a large cluster over 18 ns duration in the system with higher concentration. The oxygen-oxygen, hydrogen-hydrogen, and oxygen-hydrogen radial distribution function in a ten-water cluster showed that the position of the first and second peaks were quite similar to those of liquid TLP4P water, indicating the liquidlike structure of clustered water. As the self-diffusion coefficients, D-self, greatly depended on the time range used, the mutual-diffusion coefficients, D-mutual were also evaluated from the mean-square displacement of the center of mass of all water molecules. The D-mutual values were not so time dependent, and the values around 1 X 10(-6) cm(2)/s were qualitatively comparable to the experimental values.