Nowadays, high quality water is one of the main concerns around the world.Usually quality of reservoir outflows decreases as the water residence time is increasing. Thermal stratification and eutrophication are the most important events than can affect water quality. In the two past decades, use of mathematical models has bee increased to simulate water quality parameters and temperatures changes. The purpose of this study is to predict the potential changes in water quality in the future from common phenomena in the water reservoirs as thermal stratification and eutrophication in Marun Dam for a period of one year(march 2013-march 2014). In order to simulate the thermal stratification and water quality parameters such as NO 3 and NO 2 , PO 4 , NH 3 , TDS and DO in marun reservoir, model dimensional CE-QUAL-W2 Model was used. The inputs data includes reserviour bathymetry, meteorological data, input-output inflow and concentration of water quality parameters. Model calibration was based on vertical temperature profile and volume of the reservoir. The simulation of thermal stratification showed that it occurred in augst, september and october and Marun reservoir stratification is among thermal monomictic. Simulation of eutrophication showed that it occurred only in September and during this month, Marun reservoir condition was in Mesotrophic step. keyword: CE-QUAL-W2 model, marun reservior, eutrophication, thermal stratification