This paper describes the stress range distributions and fatigue performance of the moored floating platform “POSEIDON”. The “POSEIDON” has been set up in the Japan-sea, 41 m water depth offshore Yura port in Yamagata prefecture. The structural strains induced by waves have been measured at 12 positions of main structural members over a total of 4 years from September, 1986 to July, 1990. The fatigue damage of the structure can be estimated from the measured stress range distributions and some other methods. The fatigue analysis methods can be divided into the discrete method and the spectral method. The evaluation of the fatigue design using those methods has been made in comparison with the existing design values.