The residence time distribution (RTD) study at power generation plant was successfully carried out using radiotracer technology. In this study, Tc99m was used as the source of radioactive whereas NaI(Tl) detector was the scintillant detector. Previous study only concentrated on getting the flow rate from two successive peaks without producing any mathematical model. Thus, the hydrodynamics behaviour of the liquid inside the plant was weakly described. In this study, axial dispersed plug flow model is the best RTD model due to minimal value of sum of square which is 0.261 x 10-5 compare to other RTD models recommended by International Atomic Energy Agency (IAEA).