Abstarct: Groundwater are the most important source of water, especially in arid and semi-arid areas, as they have good and fairly constant quality and easy to use, as well. In recent decades, applying N-fertilizers in agricultural lands and sewer leakage in urban areas have caused nitrate contamination of the aquifers. Due to exictence of different sources of pollutants in Shahrood plain and importance of groundwater resources, this study was aimed to investigate spatial distribution and pollution of nitrate ion in groundwaters. Groundwater sampling was carried out in 58 operating wells at the end of May 2014. Electrical conductivity (EC) and pH were measured in situ, and nitrate and phosphate were determined in the laboratory. The results show that EC increases toward the south due to the impact of gypsiferous marl formations. It decreases in central parts of the aquifer in response to recharge from limeston formations. The values of pH decreases in urban area and the south. Nitrate concentrations vary from 1.6 to 148 mg/L, as the highest values observed in urban areas. Spatial distribution of nitrate confirms the effect of land use, as the nitrate concentration exceeds more than standard levels in urban areas due to sewer leakage and decreases in agricultural areas due to the fine texture of the soils which limits the leaching. Temporal variations of nitrate concentration rereals annual increasing rate of 1.6 mg/L. Nitrate concentration shows a reverse relationship with EC in urban and agricultural areas. It has a decreasing trend with respect to pH and depth to water table in urban areas. Phosphate concentration varies from 0.31 to 6.37 mg/L in the aquifers.