Concentration × Time (CT) value is the standard for controlling disinfection process, and reducing micro-organisms. To achieve the optimization of disinfection efficiency, all of the water treatment plants should be measured in CT values, and use one of the methods of controlling disinfectants in United States. However, despite chlorination is the base disinfection treatment for the water treatment plant in Korea, it has no regulation for disinfection efficiency. It only has a rule of monitoring 0.2㎎/ℓ residual chlorine concentration of effluent at disinfection process. The objective of this study is introducing CT values and estimating disinfection efficiency on chlorine process in the huge water treatment plant which produces approximately 730,000 ton/day to consumers by the tracer test using a solution of H2SiF6 to characterize CT values. In the worst case condition of pH 8.3, temperature 0.5℃ and residual chlorine concentration 0.9㎎/ℓ, the calculated value of CTcal/CTreq is 0.61～1.82 and T10/T value is 0.2～0.5.