Background & Objectives: Nitrate is one of drinking water pollutant which is introduced to water body from municipal wastewater. Information on nitrate concentration and its distribution in water resource is necessary in safe drinking water supply. For that reason, the present work was done for investigation of nitrate in Karaj water supply resource and its zonning with Geographic Information Systems (GIS). Materials and Methods: In this work, the nitrate concentration in 200 wells of Karaj water supply system was determined with DR5000 spectrophotometet. After that, the measured nitrate concentration was modeled by Geographic Information Systems. Results: The results show in many area of Karaj, nitrate concentration was below the available standard (45 mg/L). Modeling of nitrate concentration shows the nitrate concentration was lower in North and West North west parts of Karaj City and higher concentration was observed in Sothern eastern. However, in that area, nitrate concentration was also below available standard. Conclusion: The overall results from present work well demonstrate that drinking water source of Karaj was safe based on nitrate concentration and no health risk threat consumer health body.