The fate of CO2 hydrate disposed of in the ocean is analyzed based on thermodynamic theory. It is found that CO2 hydrate and seawater form a stable system only when (a) the system pressure p and temperature T fall within the hydrate-formation region in the ocean (i.e. p > 4.5 MPa and T < 283 K) and (b) seawater is saturated or supersaturated with respect to CO2. At ocean depths below 440 m, the pressure and temperature required for system stability are satisfied. However, since seawater is highly unsaturated with respect to CO2, the requirement for full thermodynamic stability cannot be met because the hydrate and seawater are not in chemical equilibrium and the chemical potential for CO2 in the hydrate is larger than that in seawater. Therefore, the hydrate is unstable and dissolves in seawater. Thus, CO2 hydrate disposed of in the ocean may not be a long-lived entity in the ocean as was predicted previously by many investigators. The results of our study have been confirmed by laboratory simulations.