Molecular-dynamics (MD) simulation of the diffusion of water in amorphous polyethylene has been performed. In order to obtain information about the fundamental diffusion behavior of water, the constant particle number, constant pressure, and constant temperature (NPT) MD was employed for three model systems differing in the number of water molecules and methylene group in the polyethylene chains. Time duration beyond 10 ns showed that the water molecules came together forming water clusters of various sizes. Although the self-diffusion coefficient D-self estimated from the mean-square displacement curve scattered considerably in the order from 10(-8)-10(-5) cm(2)/s, D-self for the water cluster is generally one order lower in magnitude than that of a single water. However, the linearity of the mean-square curve for the larger system which contained 4000 carbon atoms was very good, and D-self obtained for ten water molecules up to 11 ns duration showed 7.1x10(-7) cm(2)/s which was in good agreement with the experimental value.