Abstarct: Investigation of interaction between aquifers and rivers is one of the most important issues that has been considered by the hydrogeology experts. The Gamasyab River in Hamadan province with the length of about 70 kilometers originates from the Gamasyab spring at the northeast of Nahavand plain and flows toward the northwest where it enters to the Kermanshah province. Gamasyab is a perennial river. Due to the environmental importance and management of water resources, the interaction between the river and aquifer has been studied in this research. baxsed on the field studies the Gamasyab river path was divided into six parts and flow rates (discharge) were measured in hydrometery stations. In addition, water samples from river and groundwater aquifer were collected during the wet (July) and dry (October) seasons. Water samples were analyzed for the major ions and stable isotopes of deuterium and oxygen-18 in water and environment lab of the Shahrood University of technology and Iranina Mesbah Energy Company, respectively. The hydromentry measurements revealed the hydraulic connections between the surface and groundwaters as the river is mostly draining the aquifer. Water table elevation and river discharge time series were also analyzed to investigate the river-aquifer interactions and the results confirmed the hydraulic connections. baxsed on the hydrochemical studies using composite plots, Pieper diagram and correlation coefficients between major ions, the major water-type of the aquifer was determined as Ca-HCO3 which proves the major effect of surrounding carbonate rocks dissolution on the aquifer chemistry. The isotopic studies showed that except part D with the losing character, the river is mostly gaining along its path on Nahavand plain. baxsed on the results of different methods applied in the current research, the Gamasyab River was determined as a gaining river in most parts.