In order to reproduce changes of the dissolved oxygen concentration in summer in the vicinity of the Nanko thermal power plant, located in the inner part of Osaka Bay, a numerical simulation model was developed. According to the model results, daily fluctuation of dissolved oxygen concentration in the surface layer is high in June, the early stratification period, because of instability of stratification and after July. The dissolved oxygen concentration gradually decreases from a level of 6 to 17 mg/L in June to a level of 6 to 9 mg/L in July in the vicinity of the Nanko thermal power plant. Both the observations and model values near the depth of the thermal power plant outlet are 3 to 5 mg/L, which is considered not to affect the water quality of dissolved oxygen. Our simulation model should be able to reproduce the characteristics of dissolved oxygen concentration during the stratification period in the vicinity of the Nanko thermal power plant.