ABSTRACT: The hydrodynamic force acting on a fin oscillating beneath a free surface is investigated. To safely utilize floating structures under severe environmental conditions in the sea, the motion of the floating body and the mooring forces acting on the body should be reduced. To decrease these motions and mooring forces, fins attached to the structure are proposed (Yago et.al 2007). In this paper, a flat plate was used as a fin. As a first step, the hydrodynamic forces acting on the fin oscillating near free surface were resolved in new method. This method uses the modified Morison equation added to a wave damping component. INTRODUCTION The amount of natural resources is decreasing because of the industrialization of developing countries. Because the amount of natural resources in Japan is limited and most of them are imported from abroad, the utilization of resources in an exclusive economic zone (EEZ) is planned. Japan has an EEZ domain that spans more than ten times the country area and is considered to have abundant resources under the seabed. Rare metals, minerals and natural gas can be found on the sea bottom. To develop these EEZs, a large-scale platform for natural resources, renewable energy and fisheries is planned. In Japan, the technology for a large-scale platform is developed. In the Mega-Float project in 2000, a platform with a length of 1km was tested in the Tokyo bay, and a feasibility study for a floating airport was performed. Additionally, an offshore platform that is used for the utilization of an EEZ was installed in severe wind and wave conditions as compared to the platform installed in coastal region. The installation of a breakwater that protects the platform from severe wind and waves is difficult in offshore area with deep sea.