The state equation of the mercury porosimetry is formulated on the basis of rigorous thermodynamics. Expression of the state equation is Helmholtz’ free energy and it is formulating the interfacial energy due to adsorbing liquid and vapor phase mercury to the solid surface under increasing pressures. No particular pore geometry and relative arrangement of the injected mercury were assumed. The interfacial energy is based on the standard state of the mercury with free surface, and it changes by forming a new interface with an injection of mercury. As a consequence, a new concept of the effective surface area was proposed. The physical meaning of the effective surface area is the whole interfacial energy corresponding to the void capacity of pore component according to the pressure levels, and is able to predict the adsorption state in clear configuration. The effective surface area of hardened cement paste were evaluated under various water cement ratio