Abstarct: Jajarm plain with an area of about 400 km2 in the geographical range of E 56° 10' to 56° 36' and, N 37° 3' to 36° 36 ', is located in north-east of Semnan province. The groundwater extraction in this area is of vital importance. The groundwaters in this area are extracted by 109 water wells for potable and agricultural purposes. The wells are mostly located in the vicinity of the city, while southern and south-eastern parts remain relatively un-exploited. The saline water and salt plain exist in southern and south west of the plain. The main purpose of this research is to study the hydrogeological and hydrochemical characteristics of this aquifer. In order to study the hydrogeological characteristics, the data of water level variation in 16 piezometer wells of the plain is used. The general groundwater direction is from northeast and west towards the center of the plain, which indicates that the aquifer is fed from the carbonate and conglomerate highlands in the north and west of the area. To evaluate the groundwater level variation, the unit hydrograph of the plain was drawn from 1376 to 1394. During this 17 years period, a 7 meter reduction in water level was observed. The maximum reduction occurred in central part of the plane, whereas southern and northern parts of the area showed the minimum reduction. baxsed on the hydrochemistry analysis of 27 water samples from operating wells, a spring and Kal-Sour river, the groundwater quality is affected by the aquifer water withdrawal, surrounding formations and intrusion of saline water, especially in southern parts of the plain. The electrical conductivity values increase in the direction of the flow from the recharge region to the center of the area, but this trend is reversed in the southern part of the plain. The value of this parameter decreases from south toward the center of the plain, indicating intrusion of saline water in this part of the Jajarm plain.