Abstarct: Hydrological and hydrogeological studies play a significant role in energy production of geothermal resources. In this thesis, hydrogeology and hydrogeochemistry of the Sarein geothermal field are investigated. First of all, the geological and structural characteristics of the area are investigated by using mapped features in available geology map to achieve a structural map. baxsed on the chemical data from the thermal and cold springs, the hydrogeochemical properties of springs are studied, using various types of diagrams, such as Piper graph, Stiff graph, Ficklin graph, rectangular cationic and anionic graphs and water mixture graphs and their water type and sources are determined, the dominant water type is bicarbonate-sulphate which is originated from atmospheric recharge. It is also revealed that the deep aquifers of thermal springs are also suffered the mixing by groundwaters. Hydrogeological studies show that the flow direction of surface and groundwaters in the area are from West to East. These surface and groundwaters, including rivers, cold springs, aquifers and hot springs are all charged by the Sabalan Volcano foot hills, located in the west of the Sarein and their flow direction are most likely controlled by topography and faults. The conceptual model presented in this study could be useful for further development of the Sarein geothermal field for applying the geothermal resources in power generation and/or direct use.