Abstarct: Damghan plain with an area of approximately 1373 km2 is located along the Tehran-Mashad highway, between Goosheh village in the west and Qaderabad village in the east of Damghan city. Analysis of 20 years (1994-2014) water level data measured in 37 piezometers, selected from a total number of 64 piezometers on the basis of completeness of data sets- showed that the water table during this period has been steadily declining. This has led to a drop of 10.7 meters in the groundwater level from 1100.41 in 1994 to 1089.71 meters above sea level in 2014, and reduction of 734.5 million cubic meters in aquifer storage. The largest decline in the groundwater levels has occurred in northeastern corner of the plain, up to 30.28 meters and also in the central and southwestern corner of the plain. Water level drop is mainly caused by over exploitation of aquifer and to a lesser extent due to decreasing rainfall quantity in recent years. The minimum amount of drop (1.79 meters) occurred in the alluvial fan in north of the plain, at the inlet of the Cheshmeh Ali river to the Damghan plain, and also in the north-west and in discharge area in the south-east of the plain. Analysis of groundwater chemistry data in 43 selected water sampling points with the most complete data and the best distribution –from a total of 85 selected water sampling points in this plain- showed that during the years 1994 to 2014, the amount of electrical conductivity and total main ions of the aquifer has increased by 23.6 and 25 percent, respectively, while the pH of water has fallen by 7.6 percent. More than 50 percent increase in the EC has occurred in the wells which are located in the southern fringes and east of the plain (the outlet of Damghan Plain) due to saltwater intrusion from Chah Jam Kavir into the aquifer. Areas with a slight increase or even decrease in electrical conductivity are mainly located in the western half of the plain. However, due to the large area of the Damghan Plain, geological variety of the surrounding mountains and a number of local factors which control the water quality, it is not easy to determine the cause of spatial and temporal variations in the EC and other hydrochemical parameters of the aquifer. In terms of the amount and percentage changes in the hydrochemical parameters, Damghan Plain aquifer can be divided into two parts, the northern part and the southern part; the southern part is located in the outlet of the aquifer and adjacent to the Kavir. Comparison between groundwater quality in two parts indicates higher ion concentration and electrical conductivity values for southern part of the aquifer revealing intensification of the salt water intrusion and water quality degradation over time. This study also showed that the electrical conductivity of the aquifer has a significant inverse relationship with the quantity of rainfall and the elevation of the water table in the plain.