A behavior of CO₂ in the deep ocean should be clarified for the study of its storage in the deep ocean. The first thing to be considered is the formation of hydrate on the surface of liquid CO₂ (LCO2) and its growth in the direction of depth. Secondly an attention should be paid to the behavior of LCO2 which is closed and separated from a sea water with/without a barrier of hydrate. A stability of CO₂ in the ocean may be discussed taking these phenomena into considerations quantitatively. In this paper CO₂ hydrate formation was investigated experimentally at two levels of pressure, that is a medium pressure level of about 5 MPa and a high pressure level of 30 MPa corresponding to 3000 m class deep ocean complying with the pressure-temperature equilibrium diagram. Two methods of hydrate formation were applied, one is the forced mixing of LCO2 and water, and the other is the natural contact of two liquids statically and in both cases hydrate was confirmed to be formed in the pressure vessels. Dissociation of formed hydrate was also tried by relieving a pressure in the vessels. Molecular diffusions of CO₂ from the bulk CO₂ through the hydrate or directly from the CO₂ was studied by measuring the electric conductivity of CO₂ solution by using the characteristics of conductivity change by the dissolved ion in diluted CO₂ solution. From these data, three quantitative values of molecular diffusion coefficient in water, mass transfer coefficient at the interface of water and hydrate and density change of CO₂ solution were derived from the analysis of two-dimensional convection and diffusion flow by using the code which is developed for this purpose. The relative magnitude of density of LCO2 to water changes at about a pressure of 300 kg/cm², therefore two kinds of experimental conditions for direction of flow and diffusion could be represented.