In this study, energy and exergy analyses of a coal-fired steam power plant with 660-MW capacity were conducted to define the system performance. Analyses were performed with three different rates of operating loads such as 100%, 70% and 40%. Influences of three different loads on the exergy destructions were investigated for all plant components. In addition, the exergy efficiency of each component and the overall thermal efficiency of the steam power plant were computed. The first-law efficiencies of thermal power plant were determined to be 41.5%, 39.7% and 36.4% at three different loading capacities such as 100%, 70% and 40%, respectively. The second-law efficiencies of thermal power plant were calculated to be 39.1%, 37.4% and 34.3% at loading capacities of 100%, 70% and 40%, respectively. According to the obtained results, energy losses mainly happen in the condenser and exergy destructions mainly take place in the boiler.