This study was made on the corrosion resistivity evaluation method used for material selection in the Wet Limestone-Gypsum FGD system with examples of various process configuration, their corrosion environment, and the materials used in them. The wet limestone-gypsum process FGD plant is broadly divided into two types-ash-separated (dual- loop) process, and ash-mixed (single-loop) process-depending on whether the flue gas is separated from ash before being led into the absorber or led as it is into the absorber mixed with ash. Presently, the single-loop process has become the mainstream process however. The dual -loop process comprises a dedusting tower (quencher) and an absorption tower (absorber). In the quencher ash is removed with sprayed water where most of the HCI, HF etc., and a part of SOx and NOx contained in the flue gas are also removed with absorption. On the contrary, in the single-loop process which is configured of only the absorber, the flue gas is introduced into it as it is contained with ash, SOx, NOx, HCI, HF etc. The corrosion environment in these plants largely differs depending on the process type and condition. The absorber recirculated liquid has various ion inclusions among which Cl- promotes pitting corrosion and crevice corrosion while SO4’- inhibits these corrosions 9, lo). Both Cl- and S04*- cover an extremely large range between 25 to 100,000 ppm and 564 to 73600 ppm respectively, and their influence on the corrosion is related to their activity which is decided by Ca2+, Mg*+, Na+, NH+, H+ and liquid temperature. The balance of these ions is decided by the gas composition, limestone composition, make-up water and wastewater mass balance etc., of individual plants. Accordingly, materials of FGD plant are selected on the basis of evaluated results of corrosion resistivity test made under such simulated process conditions of individual plants as the characteristic flue gas composition, limestone constitution, make-up and wastewater mass balance etc., as well as field test results. In consideration of the construction cost, resin-lined carbon steel is generally used in single-loop process absorbers. But, during particular selection of materials for the single-loop process where there is high concentration of Cl-, superstainless steel or nickel alloys baving high resistivity to pitting corrosion and deposit attack are largely used. However, depending on the process condition, such austenite stainless steels as Type 304L, Type 316L (S31603), Type 317L (S31703) Type 317LM (S31725), Alloy 904L (NOS904), 254SM0 (S31254), Alloy C276 (N10276) etc., are applied as appropriate materials. Even among these, superstainless steel or nickel alloys are largely used in inlet duct, FGD fan, and single-loop process absorber where Cl- is high and SOd2- is low. Keywords : FGD, corrosion