CO2 emissions from construction of various power plants were calculated by the LCA (Life Cycle Assessment) methodology. The LCI (Life Cycle Inventory) was calculated by ＇＇NIRE-LCA＇＇, LCA software developed at the National Institute for Resources and Environment using a bottom-up approach. CO2 payback times of renewable energy electric power plants (hydroelectric, OTEC and PV) were calculated vs. conventional fossil fuel-fired power plants (coal, oil and LNG). The evaluated payback times were much shorter than the typical operational lifetimes of the respective renewable energy electric power plants.