As an offshore wind turbine facility consisted of blades, a nacelle, a supporting tower and base is subjected to strong winds as well as rough waves, so oscillations of it became larger and more complicated than a land facility, thus causing fatigue damage to the structure. Therefore we conducted time domain modal analyses for a blades-nacelle-tower-foundation system against fluctuating wind and wave loadings during twenty years, then stress time histories are analyzed to get cumulative numbers of stress ranges by two kinds of counting methods i. e. rain flow and ordinarily zero up cross. So as to compare the difference of counting methods, the analyzed relation of stress ranges and numbers is fit to a two parameter Weibull distribution. The paper concluded that the counting method such as the rain flow method can be precisely read out high frequency stress ranges piled up a low frequency stress response curve, resulting that fatigue damages of the offshore wind facility may be properly evaluated.