Trends in power costs by energy source and causes for these changes are investigated. Levelized Costs of Electricity(LCOE) are used as power costs. According to International Energy Agency(IEA) report, LCOE`s of base power increased a little in 2015 compared to 2010, whereas LCOE`s of renewable power decreased due to technical advancements. In particular, onshore wind power has reached a level that is not a big difference to base power in costs. LCOE`s of domestic power plants are analyzed according to load factors. At higher discount rates levelised fixed costs increase and so do LCOE`s. Despite of high fuel cost, Combined Cycle Gas Turbines(CCGT) turn out to be economic in case of low load factors. Prices of emission trading are used to reflect carbon costs of reducing greenhouse gas emission and effects of social costs of nuclear power, emerged after the Fukushima nuclear accident, are taken into account. Construction costs of nuclear power plants are likely to increase due to higher safety standards and so are back-end costs of nuclear fuel cycle and decommissioning costs. And coal power plants will have higher costs due to emission of environmental pollution. These changes of power costs will have impacts on energy policies and energy mix planning.