Earthquake response records of a coupled pile offshore platform were obtained in Kashima Seaberth. Perhapps, there is no data obtained on a coupled pile offshore platform at moment. In this report, vibrational characteristics of the platform are analysed by use of observed 10 records. And the results of computation by use of 1 degree of freedom model and multi degree of freedom model are compared with the results of observation. Then, added mass coefficient of piles and damping f
actor of the structure used in the earthquake response analyses of a coupled pile off-shore platform are cleared.