To study the fatigue behaviour of ship structural steels, in high stress ranges far beyond the elastic limit, the author conducted the cantilever rotatory bending tests of circular rod specimen with semicircular circumferencial notch on some kinds of steels. The time strength diagrams of low cycle fatigue, with in both logarithmic scales, are straight lines for each material. The stress ratio of the bending stress to the ultimate tensile strength is the best standard to estimate the time strength diagram of unknown material. The depth of the circumferencial crack in the ruptured section increases with the increasing number of cycles to rupture N , or the decreasing bending stress σ. We can estimate the value of N and σ from the depth of crack c . The ratio of the nominal maximum bending stress at the root of crack in the ruptured section to the breaking stress on the final area, takes a constant value unrelated with the number of cycles to rupture.