CO2 payback times were calculated for renewable energy electric power plants (hydroelectric, OTEC (Ocean Thermal Electric Conversion) and PV (photovoltaic)) by LCA (Life Cycle Assessment), using fossil fuel-fired electric power plants (coal, oil and LNG) as a reference. The CO2 emissions resulting from plant construction and production of plant construction materials were calculated for each case. The amounts of materials for construction of each power plant were taken from previous papers. The resulting LCI (Life Cycle Inventory) was calculated by "NIRE-LCA", LCA software using a bottom -up approach developed at the National Institute for Resources and Environment. The calculated total CO2 emission from the construction of each power plant was divided by annual net generated electricity. Subsequently, CO2 payback times were calculated from estimates of CO2 emissions from construction and operation of renewable energy power plants with fossil fuel-fired power plants used as the reference. CO2 payback times for hydroelectric and OTEC (100 MW) power plants were very short in comparison to PV power plants, which was mainly due to CO2 emissions construction. It was also suggested that CO2 payback times for all renewable energy electric power plants evaluated by the present paper were much shorter than typical operational lifetimes of renewable energy electric power plants.