Water is one of the most significant sources in economic- social development of human being and ecosystems. Since water purification in rivers and its transferring highly costed there is incridebly no alternative. Rivers are the main renewable fresh water resources and are critical for using in agriculture, industry and drinking sectors. Uncontrolled increase use of fertilizers and pesticides, accompanied with agricultural and industrial wastes has reduced the quality of these water resources. By continues development of industrial and urban areas, demands of stakeholders for river resources have been increased. Having focused in this study on assessing the self-purification in Tajan River along 52Km from upstream located in Shahid Rajaee dam and downstream behind of Antibiotic factory. The methodology that has been used in this project is collecting hydrometric data including water level, water temperature, velocity, and flow from hydrometric stations which are provided by Mazandaran Regional Water Company and also current status of concentration of BOD, COD, pH and DO measured in laboratory of Department of Environment (DOE) of Mazandaran. These stations are located manily close to three important industrial areas. Qual2K- river water quality modelling software is used. This model might be used as a powerful instrument in monitoring the quality of rivers along the whole path. Calibrating and validating done with a series of data in 1393. Self-purifying potential of the river evaluated based on the ten years average flow and finally based on these results, the critical segments along the river were identified. Thus 3 scenarios considered to assess water quality management, including: decreasing water flows from upstream, controlling discharge of drainage systems and industrial waste and contaminants and at the end artificial strategy for increasing the level of dissolved oxygen with a cascade weir assessed by using CCHE-2D. Evaluation of artificial aeration showed that this strategy is not remarkable to mange water quality. Hence other alternatives such as controlling discharge to the river would assessd in winter season, without agricultural discharge at 9 centigrade. With this in mind all these precedures 2.3 times were influenced on self purification in upstream. To be in a stable condition in Tajan river sugessted that padyland and gardens must decrease its discharge up to 60%. Keywords : Modeling, Self purification, CCHE-2D, Tajan River, Discharge