A steam power plant can run at off-design due to change of ambient conditions or load demand. This needs to study how part-load affects the power plant performance. In this paper, the results of energy and exergy analysis carried out on a 65 MW Derna steam power plant in Libya at part load conditions are presented. The article aims to identify the magnitude, location and source of thermodynamic inefficiencies in the steam power plant at part loads. The performance of the plant was estimated by a component-wise modeling and a detailed break-up of energy and exergy losses for the considered power plant. The required outputs (power, heat and exergy destruction) of the various components and for the whole plant were assessed and calculated using mass, energy and exergy balance equations of the developed model. Based on this model, a computer program was written and used to investigate the performance of the power plant. According to the results, for full and part loads, the largest amount of energy loss occurs in the condenser and the least occurs in the boiler. In terms of exergy, the major exergy destruction was found to be maximum in the boiler, followed by the turbine, and then the condenser. The results revealed remarkable dependency of overall energy and exergy efficiencies, total exergy destruction ratio, heat rate and specific fuel consumption on the change in the load of the power plant. In conclusion, the results show how energy and exergy have been used to locate places of inefficiencies in the power plant operating at different loads.