The purpose of this paper is to investigate the mechanical properties of concrete beams in water quantitatively. 　For this purpose, static and cyclic fatigue tests of concrete beams had been done in water or in air. In these tests, the mechanical properties of the beams in water were compared with those in air. 　The results are shown below: (1)The static loading capacity of the beams in water and that of in air are almost same. (2)The cyclic loading capacity of the beams in water is 10-20% less than that of in air. (3)With lap splices. the static and cyclic loading capacity of the beams in water is about 25% less than that of in air.