With considering the increasing of global temperature, and also the concern of global climate change, many policy makers worldwide have been accepted the importance of reducing greenhouse gas emissions, in particular from the power industries. Energy resource use is one of the most important and contentious issues of our time. The ocean provides a vast source of potential energy resources. Of the total solar radiation, oceans are the largest collectors, accumulating 250 billion barrels of oil equivalent, according to an estimate. This vast amount of solar energy absorbed in the oceans can be converted into electricity by a process known as Ocean Thermal Energy Conversion, popularly known as OTEC. OTEC makes use of the difference in temperatures of warm surface water (22–27 °C) and very cold water at a depth of 1 km (4–7 °C). an open-cycle plant based on creating a rising mixture of water and steam bubbles or “foam”, which is separated at a height above sea-level, such that the water can be used to drive a turbine rotor. In closed-cycle OTEC, warm seawater heats a working fluid with a low boiling point, such as ammonia, and the ammonia vapor turns a turbine, which drives a generator. This paper discusses about the ocean energy, ocean thermal energy potential, ocean thermal energy conversion by the close, open and hybrid cycles, environmental impact and special conditions of these process.