In order to reproduce the physical characteristics in the vicinity of the Nanko thermal power plant located in Osaka Bay, a semi-enclosed coastal bay, in summer, a numerical simulation model was developed. The horizontal and vertical distributions of water temperatures were fitted to the observed and model results, although the calculated salinity values are slightly lower compared to the observed values. This model can reproduce the physical environment in Osaka Bay and the Nanko area. Calculation results showed that the discharge from the power plant may lead to a disturbance of the stratification within the area of 500m away from the power plant discharge.