Equilibrium water sorption on Nylon 4 and diffusion process of water in the polymer were studied. Water sorptive ability of Nylon 4 is lower than that of regenerated cellulose, higher than that of cotton and three times of that of Nylon 6. The temperature dependence of the sorption isotherm is not significant.Diffusion coefficient obtained from the permeation experiment depends remarkably on the water content, which is common for the lyophilic polymers. In the case of water content 0.19g/g, Arrhenius_??_ plot of diffusion coefficients has an inflection at about 26°C, which is lower than the glass transition temperature. Sorption rate curve is not of Fickian type even at temperatures higher than the glass transition determined by dilatometry.