Simulation was performed to optimize a flue gas desulfurization (FGD) system, which removes sulfur dioxide (SO2) from flue gas. Optimum conditions were Sought by studying the influence of various parameters on desulfurization efficiency and CaCO3 availability for the limestone-gypsum method (LGM) in comparison with the magnesium hydroxide method (MHM). When an FGD system with a cross-sectional area of 30 m(2), an absorption tower of height 15 m and a water depth in the oxidation tank of 5 m was used to treat flue gas containing 1,000 ppm of SO2 at a flow rate of 500.000 m(3) (STP).h(-1), we obtained the following optimum values: dissolved SO2 concentration: under 0.001 kmol.m(-3) (LGM) 0.05 kmol.m(-3)(MHM). diameter of droplet: 1.5 mm; pH: 6.5 (MHM), solid CaCO3 concentration: 1.0 wt% (LGM), height of absorption tower: 15 m (LGM), 10 m (MHM);, and liquid flow rate of recirculation: 6,000 m(3).h(-1) (LGM), 4 500 m(3).h(-1) (MHM). Next, we Studied the control system for temporary fluctuations of SO2 gas concentration and the flue gas flow rate by Using the above design and operating conditions obtained for LGM. As a result, it was indicated that the stable maintenance of desulfurization performance was possible by the feed-back control using a PID controller.