Nitrogen stable isotope composition (δ ¹⁵N) of nitrate (NO₃^-) deposition can aid in source apportionment of its precursor emissions, nitrogen oxides (NO_x), with implications in mitigation policy to address serious air pollution. However, potential δ¹⁵N fractionation during atmospheric NO₃^- formation may hinder accurate quantification of NO_x contributions. Previously, NO_x photochemical reactions have been suggested to be the dominant δ¹⁵N fractionation process in NO₃^- formation. Here, we have quantified the potential fractionation effects associated with NO_x photochemical reactions based upon δ¹⁵N‐NO₃^- measured in 1 year of daily‐based bulk deposition samples in Shenyang, a megacity with distinct seasonal fossil fuel combustion in northeastern China. The mean δ¹⁵N was 0.9 ± 4.0‰ and ranged from ？4.9 to 8.3‰, with the lowest and highest values observed during summer and winter, respectively. Calculated NO_x photochemical equilibrium fractionation may account for up to 42% of the observed seasonal δ¹⁵N change in NO₃^- deposition. However, the relative NO_x source contribution trends, estimated based upon a δ¹⁵N Bayesian mixing model, were insensitive to NO_x photochemical fractionation considerations. Overall, NO_x source partitioning based upon averaged year‐long δ¹⁵N of bulk NO₃^- deposition estimated the following relative trend in NO_x emission sources: coal combustion > biomass burning = vehicle emissions >> soil emissions. Seasonally, the increase of coal combustion emissions from summer to winter drives the seasonality of δ¹⁵N in NO₃^- deposition, indicating a necessity to control NO_x emissions from coal combustion to improve wintertime air quality.