The existence of water in capillary pore affect the strength development and durability of concrete.The state and phase change of capillary water is explained by thermodynamics. In this study we proposed the modified mercury porosimetry to estimate the capillary pore structure and the equation that described the thickness of adsorbed water by relative humidity and pore radius under the equilibriumConsequently the moisture desorption isotherm was estimated by relative humidity and pore structure measured by mercury porosimetry