Traditionally main driving force of chloride penetration in concrete have been regarded as diffusion, however, large construction such as tunnel elements and foundations for bridges can be placed on large sea water depths where the water pressure is fairly different from the pressure in the splash zone. Moreover, hydrostatic pressure increases in proportional to measured depth from the surface of water because of the increasing weight of water exerting downward force from above. In this study, new experiment method to estimate chloride penetration of concrete under water pressure is introduced. When the life length for such a concrete structure on a large sea water depth is estimated using the normally used models, the concrete test specimen.