In order to increase the ocean space exploitation by using the floating structures, it ceems necessary to develop the tension mooring system. This system has many advantages such that it becomes relatively inexpensive with depth, suppresses the motions of the floating structure and needs only minimum mooring spaces. An at-sea experiment is planned to establish those fundamental technologies. The semi-submersible consists of twelve columns with footing, and its principal dimensions are 30 × 20 × 12.5 m. This is supposed as a proto-type of bigger scale floating platform. The structure will be moored mainly by the six catenary chain lines with anchors. Further mooring by the four tension legs will be tested. The tension leg is made of rubber and chain. This composite material has the higher breaking load of chain and also anti-impact and corrosion resistance of rubber. In this study the tension mooring system has been designed to match with the environmental conditions of test site. The mooring tension characteristics have been estimated both by the model tests and the theoretical calculations.