Offshore concrete structures shall be received two-way repeating loads in waves. In this report, failure pattern and fatigue strength of reinforced concrete beams subjected repeating loads are discussed. Test beams are double-reinforced concrete beams varied concrete strength, ratio of steel, ratio of stirrup and so on. For loading of test beams, electro-hydraulic fatigue testing machine were employed.
　Results obtained from fatigue tests are as follows.
1) Failure patterns of testing beams are affected by the effects of ratio of stirrup and concrete strength.
2) Difference of fatigue strength between two-way loading and one-way loading is explained. with maximum stress amplitude of reinforcing bars.
3) Fatigue strength curve of reinforcing beams are obtained at 1.0×106 and 2.0×106 times loading.
4) Residal cracks remains in reinforced concrete beams by repeating load and the crack width obtained from experiment are almost equal to the computed values from Okamura's Formula or PHRI's Formula.