Abstarct: Chemical composition of groundwater is affected by different factors such as climate, geology, hydrogeological characteristics of the aquifer and anthropogenic factors like irrigation return flow from agriculture activities. Mojen plain, which is located Shahrood city in Semnan province, has expericed a development in agriculture so, contamination of aquifer is probable. This study has aimed to asses quality of Mojen aquifer with emphasis an agricultural pollutions. Hydrogeology and hydrochemistry of aquifer was firstly investigated by data from observation wells. Seventeen agricultural wells were also sampled at Tir 1391. Electrical Conductivity (EC), pH and Tempereture were measured in the field. The samples were transported to laboratory for measurement of major anions and cations (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO4-) and main agricultural pollutants of Nitrate and Phosphate. According to the Iso-potential map, aquifer is recharged from west and northwest. The General flow direction is from West to East. And discharge area is located at the South-Eastern parts.water table elevation has been increased in some observation wells located near the agricultural area. Implying recharge of aquifer by excess irrigate return flow. Anionic water type at aquifer is HCO3-, which is change to Cl- and SO42- at the North due to the outcrope of the shale, Marl and evaporiate sediments. Highest values of EC, SO42- , Cl- and Na+ are found to be in the Northern parts of aquifer. Groundwater resources in the agricultural areas, show high values of NO3 - and PO43-. There is a clear relationship between EC and NO3- and also between NO3- and the depth at the wells. Nitrate and phosphate with the mean concentration of about 63.7 and 0.26 mg/L, respectively are high in wells with depth less than 175 m, implying their surfacial origin, confirming detroirated impacts of agrigulture on quality of Mojen Aquifer.