In this paper, the influence of sea water as mixing water on the long term durability of concrete under marine environment is studied. For this purpose, many specimens have been exposured for twenty years at tidal zone of the experimental pool which simulate the ebb and flow of the natural sea condition.
　The experimental results obtained are as follows;
(1) The concrete strength after 20 years' exposure is not affected by the mixing water.
(2) The corrosion of bars embedded in concrete after 20 years' exposure is not affected by the mixing water.
(3) The amount of Chloride ions measured in concrete after 20 years' exposure is not affected by the mixing water.
　Therefore, it is concluded that the influences of sea water used as mixing water on long term durability of reinforced concrete stuructures are negligible under marine environment.