To carry out a reliable design of the ocean project and same thing for observation of marine science, it is essential to survey the ocean environment extend over a long time and continuously. Japan Marine Science and Technology Center (JAMSTEC) proposes to construct an ocean stable laboratory (OSL) in order to enable the long term continuous research and to contribute the development of the deep ocean area far from the shore. The proposed OSL is planned to be a kind of tension leg platform (TLP) which suppresses its vertical motion (heave, surge, sway) by the leg tension and it is stable and removable. The principal particulars of OSL are length of 77 m, breadth of 77 m and depth of 70 m and to be set in a deep ocean area of about 800 m water depth. This OSL equips various kinds of facilities for experiment and research works, the support systems for undersea work, the measuring and monitoring systems, the telecommunication network, the accommodation facilities, etc. Around this OSL, the undersea habitation, the underwater shuttle boat, etc. are also arranged in the plan to perform the research works. JAMSTEC have been performing fundemental studies of the TLP and also performed the sea test of TLP in 1986 and 1987 at Yura, Yamagata prefecture and confirmed its availability and safety. Based on the results of the above, design procedure of the optimum hull form of the OSL is developed in this study. The model experiment using 1/100 sacle model is also performed to confirm the motion characteristics and varying tensions of the tendons in waves. This paper discusses the design procedure of an optimum hull form of the OSL and the results of the model test.