Abstarct: Salinity is one of the most important factors affecting the depredation of groundwater quality which has become a growing problem in recent years. Electrical conductivity has extensive range and specific dispersion pattern in the Shahrood aquifer. The amount of this parameter changes from about 600 μs/cm in the northern and western parts to more than 11200 μs/cm in eastern and southeastern parts of the aquifer. The sudden changes of electrical conductivity in the Shahrood aquifer indicates hydrochemical discontinuity in this aquifer. Although the general flow direction in the aquifer is from the eastern, northern and northeastern to the western areas, the creation of groundwater divide line due to concentration of pumping wells and focused discharge of these wells caused local changes in groundwater flow direction. In the opposite direction of the general flow direction of groundwater in the aquifer, as a result, a part of inflows from the eastern and northern boundaries in central area deviate toward the east. In present study, in order to identify the source of groundwater salinity in the Shahrood aquifer, a total of 115, 40, 33 and 3 groundwater samples were collected and analyzed to determine major elements, trace elements, stable isotopes and 3H isotopes respectively. Determination of the source of salinity in the Shahrood aquifer has been carried out using 4 approaches including hydrochemical techniques, statistical analyses, geochemical inverse modeling, and Tritium isotopic tracers and age dating. On the basis of the results have been achieved with ment ioned approaches, dissolution of evaporative minerals (halite and gypsum), calcite precipitation, silicate weathering and cation exchange are the most important factors in increasing salinity in this aquifer. According to the isotopic analysis results, the salinity mechanism of the Shahrood aquifer has been determined as follows: The old meteoric water or water with meteoric origin, which originated from higher and colder elevation and is more depleted than the freshwater aquifers in terms of isotopic content as well, is the source of saline water in the Shahrood aquifer. The water divide line along with decreasing the groundwater flow velocity, caused a reduction of the hydraulic connection between eastern - southeastern inflows and western parts of the aquifer. Decreasing the groundwater flow velocity, accompanying increasing the water residence time and consequently its age, provide the condition for more dissolution of minerals and more increase in the salinity as a result. Dissolution of gypsum mineral lead to the enrichment of 18O and shifting the samples location to the right hand of the Shahrood meteoric water line (SMWL) as well.