When sewage and industrial wastewater are treated to improve the coastal water quality, we usually consider the reduction of BOD and/or COD. Due to the inflow of untreated nitrogen and phosphorus, however, the coastal water has eutrophicated. Thus, to improve the water qualify, it is necessary to treat and reduce nitrogen and phosphorus which are the factors limiting algal growth. Hence, with regard to the adaption of Ocean Outfall for the treatment of wastewater in Suyoung Bay, we studied the effectiveness and environmental assessment of Ocean Outfall of which cost is lower than that of advanced treatment, The effectiveness of Ocean Outfall of the second treatment effluent in Nambu S.T.P. was simulated, using the eco-hydrodynamic model as far field model the result showed that the water quality of coastal area receiving wastewater on the surface sea has improved. But the concentrations of nitrogen and phosphorus around the diffuser of Ocean Outfall system can cause many problems on estuary, such as red tide, eutrophication and aquatic toxicity.