Water-soluble organic nitrogen (WSON) plays an important role in atmospheric chemistry and climatic change. However, few studies have focused on WSON in size-segregated particles. In this study, size-segregated particle samples in the size ranges of <0.49 mum, 0.49-0.95 mum, 0.95-1.5 mum, 1.5-3.0 mum, 3.0-7.2 mum, 7.2-10 mum were collected in Guangzhou, China from September 2014 to July 2015 by a high-volume sampler. Samples were analyzed for water-soluble total nitrogen (WSTN), WSON and and water-soluble inorganic ions (WSIN). The concentrations of WSON in each size range showed a similar seasonal trend with higher concentrations in autumn and winter and lower concentrations in spring and summer. The seasonal average concentrations of WSON in PM3 ranged from 1.15 to 2.62 mug/m~3, contributing to 63%-71% of the WSON in PM10. The concentrations of WSON in particles <3 mum exhibited a unimodal distribution in four seasons, enriching in the 0.49-0.95 mum particles. The principal component analysis (PCA)/absolute principal component scores (APCS) analysis showed that WSON can be mainly attributed to the burning of local fossil fuels in particles < 0.49 mum, to construction dust and photochemical oxidation leading to its secondary formation in 0.49-0.95 mum particles; and to photochemical oxidation leading to secondary formation alone in 0.95-1.5 mum particles. Our research improves our understanding of the environmental geochemical processes of organic nitrogen compounds.