In this article the method of cost optimization of the "Wet Limestone Flue Gas Desulfurization System" is presented. The optimization calculations include process and cost models. The process model describes the most important stage Of SO2 removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO32-, dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO2 removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue Gas desulfurization plants.