This study was carried on hydrocyclones in the Wet Flue Gas Desulfurization (WFGD) system of a local thermal power plant. In WFGD systems, hydrocyclones are used for classification in terms of PSD of limestone, dewatering the gypsum slurry and recycling the wastewater. Seperation efficiencies of hydrocyclones (waste water and gypsum) in power plant were calculated referring to each hydrocyclones’ inlet size of D25. Results obtained with Malvern Mastersizer for the samples from each exits of hydroclones were taken into consideration. Separation efficiency for waste water hydrocyclone was calculated as 4.0 % while it was calculated 77.5 % for gypsum hydrocyclone.