From the results of tracer test for the existing clearwell in Y water treatment plant, T10 and T10/T were calculated as 150 min and 0.24, respectively. Therefore it required the modification schemes for improving hydraulic efficiency, surrogated by T10 and T10/T, and disinfection performance. In this study, using transient CFD(Computational Fluid Dynamics) simulation technique, tracer tests on dynamic condition for the suggested schemes were simulated. From the results of simulation, it was revealed that 8~6 baffles are necessary to guarantee the disinfection ability in the existing clearwell. Also, installing orifice baffle in the vicinity of inlet could increase plug flow fraction within clearwell.