Nitrate (NO3 (-)) pollution is a severe problem in urban aquatic systems especially within megacity undergoing rapid urbanization, and mostly, sewage is supposed as the prevailing NO3 (-) source. A dual isotope approach (delta N-15-NO3 (-) and delta O-18-NO3 (-)) was applied to explore the variation in NO3 (-) isotopic signatures in sewage processed by wastewater treatment plants (WWTPs) in Beijing from 2014 to 2015. We found that the raw and treated sewage owned the different NO3 (-) isotopic signatures, including delta N-15 from 1.1 to 24.7 aEuro degrees and delta O-18 from 1.6 to 22.8 aEuro degrees in raw sewage, as well as delta N-15 from 6.1 to 22.8 aEuro degrees and delta O-18 from 1.6 to 13.2 aEuro degrees in treated effluents. The WWTP processing would result in the enrichment of NO3 (-) isotopic compositions in discharged effluents with NO3 (-) concentrations increasing. Besides, advanced sewage treatment technology with more pollutant N reduction may raise the heavier NO3 (-) isotopic compositions further. The NO3 (-) isotope value ranges of urban sewage and manure should be separated, and the seasonal and tighter NO3 (-) isotope value ranges are supposed to improve the accuracy of source apportionment. The NO3 (-) isotope value ranges conducted in this study might provide useful information for tracing NO3 (-) sources towards the implementation of efficient water pollution control in Beijing.