In general, hydraulic model tests and numerical simulations can be used for securing enough informations in order to assess the environmental impact by thermal discharge after the power plant operation. However, the numerical simulations should be verified by the consequence of hydraulic model tests or the field data. Then, the numerical model can be used as a prediction model to foresee the nature of thermal diffusion when the additional generators will be operated. The third nuclear power plant in Taiwan has been constructed in 1984. In order to protect the abundant corals which distributed on the rocky bottom around this power plant, a complete studies on thermal diffusion have been performed, accordingly, a verification with field measurements were also made in this paper.