Background & Objectives: Nitrate is an important ion for evaluation of drinking water quality. Nitrate concentrations higher than the permitted limit is an indication of water pollution to urban, agricultural, and industrial wastewater. Consumption of water, containing nitrate levels of higher than the standard values, adversely affects human health. This research performed to determine nitrate concentration in drinking water supply sources and distribution network of Ardabil city during four seasons of 2008-2009. Methods: In a descriptive cross-sectional study, nitrate levels were monitored in 34 samples from 17 wells supplying drinking water of Ardabil city, 44 from distribution network, 4 from water treatment plant effluent, and 4 samples from water reservoirs during the four seasons in 2008-2009. Analysis of nitrate ion was done according to standard methods. Results: The average concentration of nitrate in wells and distribution network were in a range of 7.095-47.08 and 3.05-57.62 mg/l, respectively. Respective values for nitrate concentration in reservoirs and treatment plant output were on average 9.95 and 12.31mg/l. Conclusion: Nitrate concentrations in all samples (except one sampling location in distribution network) were below the national standard levels set by Iranian Institute of Standards and Industrial Research. However in a well the nitrate concentrations were very close to the Maximum Contaminant Level.