The Ocean Thermal Energy Conversion (OTEC) is refocused as available option for securing a sustainable and environment friendly source of energy. OTEC is a system for generating electricity using the temperature difference between the shallow and the deep water of the ocean. The thermal energy of the ocean is more stable and insusceptible to climate than the other natural resources such as the wind and the ocean waves. This makes the OTEC systems as being capable of generating electricity more constantly than the other renewable energy conversion systems and put the system along with the geothermal power systems at the top of the list regarding the stability and constancy of power generation. In addition, the seawater used in the OTEC power plants can also be used for many other purposes such as the seawater desalination, house cooling and aquaculture. This paper presents a review of the OTEC resource evaluation method. OTEC may have the highest potential of the available ocean energy options. In the case of solar power absorbed by the oceans, the OTEC potential is about 1.1PW by Uehara and Vega. On the other hand, Krishnakumar and Nihous show that estimated global annual maximum OTEC net power is about 7TW using the ocean general circulation model MITgcm. Recently, OTEC potential in the waters around Japan is evaluated using the data of temperature difference between warm surface seawater and cold deep seawater for the five years from 2005 to 2010. The OTEC potential is calculated by integrating the energy density of Japan’s EEZ and territorial waters. Moreover, estimate conditions include geographical condition, ocean temperature difference, installment density, and utilized capacity. The introductory potential of float type OTEC, the inside of Japan’s EEZ and territorial waters, is 174GW when utilized ocean temperature difference is 20°C or more.