We evaluated the energy balance of an ocean thermal energy conversion (OTEC) system that is one of the substitute energy sources. It was found that a 100 MW OTEC system reduces the amount of CO 2 emission by 140,000 t-C/year, as compared with a 100 MW coal fired power plant. Its energy pay back time was calculated to be 0.46 year. In addition, the synergistic CO 2 uptake effect by the compulsory circulation of the ocean was calculated to be 7,800 t-C/year for a 100 MW OTEC system on the basis of the difference between the present and pre-industrial CO 2 concentrations in the atmosphere.