This paper deals with the increase of cost effectiveness in the design of thermal power plants and presents a combination of the iterative exergoeconomic optimization technique with fuzzy inference systems (FIS). The development of FIS for the iterative exergoeconomic evaluation and improvement of plant components is described. An application to the iterative cost minimization of a simple cogeneration plant is discussed. Several case studies show that the application of fuzzy logic in exergoeconomics is very useful.