The tow-line S-N diagrams are recently used in the offshore structure design curve as seen in the UK DOE offshore installation ; guidance on design and construction (1984). In the present paper, fatigue damage calculations are conducted for the two-line S-N diagrams, and the effects of the gradient of the S-N diagram in a high cycle range on fatigue damage are evaluated. In the result, simplified evaluation methods for fatigue damage are proposed, where the use of numerical integrations are not required. The advantage that the fatigue strength of high tensile strength structural steel welded joints are higher than that of miled steels in low cycle range is evaluated from the view point of fatigue damage. Because the calculation of the fatigue fracture probability Pf is important in evaluation of reliability of offshore structures, a new and reliable calculation method is developed, and relationship between the fatigue damade D and the fatigue fracture probability Pf is shown.