The Burning of fossil fuels, such as coal, fuel oil, natural gas, and diesel in power sector, has become one of the major sources of air pollutants, namely COX, NOX and SOX. These are thought to adversely affect the environment and human health. This paper assesses the external cost of electricity production from fossil fuel power plants in Iran using the Extern-E study. In addition, the future of average amount of emissions and their external cost was predicted according to the development of Iran's power generation system. Investigation of power generation planning in Iran from 2013 to 2023 shows that the average emissions of PM10, SO2, CO, and CO2 will reduce by 26.7%, 54.2%, 65.4%, and 20%, respectively, in fossil power plants. Moreover, it was predicted that the average external cost of power generation in 2023 would decrease up to 28.9%, 27.6%, and 23.1% in low, medium, and high scenarios, respectively. Moreover, analyzing the trend of Iran's electricity exportation and considering the prospect of Iran's energy sector, we claim that substitution of old gas power plants with high throughput combined cycle will help achieve the goal of Iran's power generation development more sustainably.