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Nitroaromatic compounds (NACs) are an important class of nitrogen-containing compounds in fine particles. The investigation of characteristics and seasonal variation of NACs in PM 2.5 increases our knowledge about nitrogen-containing compounds and contributes to the scientific basis in formulating reduction policies of NO x in urban areas. In this study, we analyzed the chemical composition of PM 2.5 samples collected from March 2018 to February 2019 in an urban location in Shanghai. A total of 2439-3695 organic molecular formulas were detected using UPLC-Orbitrap MS. Nine NACs were quantified using an internal standard method. In spring, ρ (NACs) ranged from 3.12 to 16.76 ng·m -3 , and the average concentration was 9.31 ng·m -3 . In summer, it ranged from 1.05 to 9.70 ng·m -3 , and the average value was 4.16 ng·m -3 . In autumn, it ranged from 2.87 to 36.27 ng·m -3 , and its average was 9.84 ng·m -3 . In winter, it ranged from 4.83 to 56.23 ng·m -3 , and the average was 22.37 ng·m -3 . 4-Nitrophenol accounted for more than 25% of the quantified NACs in different seasons. In summer, the concentration of 5-nitrosalicylic acid accounted for 36%, but it decreased to 19% in winter. NACs in summer mainly originated from secondary formation, as evidenced by their clear correlation with the oxidant level, whereas biomass burning became the main source of NACs in winter.