In this study, the effect of hygrothermal conditioning on the moisture diffusion properties of the fabric composite (glass fiber/epoxy resin) was investigated. The water uptake of the specimens conditioned in humid environment at different relative humidities (0, 60 and 96 % r.h) at constant temperature (60°C) was evaluated by weight gain measurements. The moisture diffusion properties of the fabric composite (glass fiber/epoxy resin) were determined using standard weight gain method. The weight gain experiments were performed to determine the equilibrium moisture content Mm of the fabric composite as a function of relative humidity (r.h). The measured weight gain is then fit to the solution to the diffusion equation (Fick’s law) to determine the diffusivity D. The comparison carried out between the values obtained of the characteristic parameters (D and Mm) of the kinetics of water absorption by the hygrothermal test of conditioning carried out into the laboratory and those given by Loos and Springer confirms the following principal remarks clearly: the diffusion coefficient D and the maximum weight gain Mm depend not only on the nature of material but also of the environmental conditions (hygrothermal conditioning). The maximum concentration of water (matrix+interface) obtained from calculations based on measured values, where a homogeneous diffusion phenomenon is assumed inside the material (Df=0), shows clearly that the presence of fibers in a polymeric matrix reduces the water up-take of the matrix by about 4 times.