In this article, energy, exergy, and environmental (3E) analysis of a 400 MW thermal power plant is investigated. First, the components of the power plant are examined in terms of energy consumption, and subsequently the energy losses, exergy destruction, and exergetic efficiency are obtained. It is shown that the highest energy losses are in the closed feedwater heaters Nos. 1 and 5 and the boiler with amounts of 7.6 × 10 J/s and 6.5 × 107 J/s, respectively. The highest exergy destruction occurs in the boiler and amounts to 4.13 × 108 J/s. The highest exergetic efficiency is 0.98 and is associated with the closed feedwater heaters Nos. 4 and 8. It is observed that the exergetic efficiency and exergy destruction in the boiler are the primarily affected by changes in the environmental temperature. Furthermore, by increasing the main pressure in the turbine, the load on the power plant is increased, and increasing the condenser pressure reduces the load on the power plant. In an environmental analysis, the production of pollutants such as SO2 production and CO2 emission has been investigated.