Model experiments and numerical simulations on motions and mooring tensions of an ocean observation platform were made for the design of the mooring system and platform. The platform is under development for the study of green house effects and is planned to be moored in very rough sea area of 5000m depth. The performance of wave power device, which is installed in the platform to supply the electric power for instruments used in the chemical, biological and physical measurements, is also discussed based on pressure variations generated in the air chamber of the platform model. In general, results of numerical simulations showed good agreement with those of experiments, though a few points needing improvement were discussed.