Abstarct: Miami plain is located in the eastern part of Semnan province. In the last few years, the both water level and quality of the aquifer has continuously declined due to the high groundwater extraction. In order to investigate the hydrogeochemical-isotope and origin of salinity of the groundwater resources of the Miami plain, the piezometric water level as well as 47 water samples taken from the exploitation wells in the plain have been measured and used. The general groundwater flow direction is from the southwest to the north of the plain. In the central part of the plain, the rise of the groundwater level has occurred, indication recharge area. The reason for this could be the rise of bedrock or due to recharging through the hidden fault path. The discharging area of the plain are mostly through pumping wells in the northern part, which also happens to a lesser extent in the eastern and western parts of the plain. Electrical conductivity values vary between 600 to about 10,000 μS/cm, which have almost the same trend as the groundwater flow direction. The most of saline water intrusion occurs from the northern part of the plain, lead to enhancing the EC value in this part to more than 7000 μS/cm. This northern interned salty water is mixed at the central of the plain with the recharged fresh water from the southern part. The dominant water type of the samples is Cl-Na-Ca to HCO3-Na. All the samples are saturated to supersaturated with respect to halite and gypsum and saturated to supersaturated with respect to dolomite and calcite. The hydrogeochemical and isotopic studies of the study area indicate that the waters of the study area are of atmospheric origin and mainly local precipitation is of Mediterranean and polar origin. The isotopic data of the studied water sources are mainly lighter than the rains of the region and have a similar behavior to the snow samples. Therefore, it can be concluded that recharged water in this area is mainly done in the form of snow. The most probable source of salinity is the mixing process between salty water with a enricthment isotope value and fresh water with a lighter isotope. The salinity source of the water samples does not have a deep origin; rather, the infiltration of salt water probably took place from the upper laxyers and from the side of the aquifer, and the salinity source is located in the upper laxyers of the unsaturated zone in the area.