Groundwater is the main source of water supply in many plains in Iran, including Mashhad plain. With increasing population and urban, industrial and agricultural activities, groundwater is exposed to pollution. Nitrate is one of the main contaminants in groundwater. The purpose of this study was to determine the nitrate pollution level and its origin in Mashhad plain. Fourty water wells were selected in different agricultural, industrial and urban regions of Mashhad plain. Sampling was performed monthly, for six months of July - December 2001. N03-, EC, pH, TDS, C03-2,HC03-, cl-, S04-2,Ca, Mg, Na, K, BOD, COD and Coliforms were measured and compared with different standard limits.The results showed that nitrate concentration in some parts of the studied area was more than the maximum permissible level (45 mg/L). Mean nitrate concentration varied between 5.3 to 74.4 mg/L. Highest nitrate pollution was in urban and populated parts of city of Mashhad. Water wells in agricultural and industrial areas had desirable quality. Mean EC of water samples varied from 0.4 to 3.2 dS/m, TDS from 230 to 1995 mg/L and total hardness from 152 to 1037 mg/L. Nitrate concentration in Jomhori square, Seyedi and Sheshsad-dastgah areas were 74.4, 68.5 and 49 mg/L, respectively, and its origin was due to leakage of domestic wastewater into the groundwater.