The water diffusivity of the fuel cell electrolyte membrane is the most important factor that determines the ionic conductivity of the membrane. Due to the existence of electro-osmosis that is a water-permeable driving force, and reverse diffusion that is a non-linear region, depending on the operation conditions of the fuel cell, an accurate understanding and calculation method of moisture permeation of the membrane is required. The moisture diffusivity of the electrolyte membrane at the temperature and humidity can be determined by the water diffusion coefficient. Firstly, diffusivity of water is calculated with measured water contents, Experiments are carried out over temperature and humidities of gases.