During the past, most of the off-shore structures have been constructed either of steel or concrete. Major advantage of concrete structures is its corrosion resistance without painting work and consequent low maintenance cost, while those of steel structures are light weight construction, short building time and practical use of facilites in shipyard etc. So, if it is possible to apply prestressed concrete to the underwater hull together with steel to the other part of the off-shore structures, both concrete and steel will serve with their features at its best respectively. In this paper, the authors carried out statical load test, repeated load test and watertight test with partial, full scale model of three kinds of steel-concrete joint and also statical load test with a whole prototype model of composite structure. From these experiments and analysis, following conclusions are obtained. 1) Joint structures between steel and concrete have sufficient durability for expected repeated load and have enough water tightness at ordinary water pressure. 2) The calculated results by steel-concrete composite beam theory for bridge were in good agreements with experimental ones. It was clarified that the theory is also applicable to offshore composite structures. 3) Concrete and steel worked together in a body up to the destruction of models. And these composite structure have huge absorbing power of energy. 4) Influences upon beam strength by concrete joint were negligible. Therefore pre-cast block building method will be applicable to off-shore composite structure construction.