Abstract: To determine the effective moisture diffusion properties of fiber polymer-matrix composites, a new solution method was established by using a concept of unit cell. The effects of temperature and fiber volume fraction on the effective moisture coefficients were investigated. In this study, the impermeable fibers were assumed to be distributed uniformly in matrix. Using the proposed model, effective moisture diffusivities were calculated at different temperatures and fiber volume fractions. The results show that effective diffusivities of the composites increase with the increase of temperature, and decrease with the increase of fiber volume fraction. Effective diffusivities of regular hexagonal composite are a bit larger than those of square one under the same temperature and volume fraction. The numerical results are compared with those from empirical formula, and good agreement is achieved. Therefore, it proves that the present method is valid and reliable to predict moisture diffusion properties for fiber polymer-matrix composites by the unit cell model. The present results are significant to explore moisture diffusion mechanism and overall properties of composites.