The optical properties of black carbon aerosols (BC) are determined by the particles size and the associated non-BC materials, which may be source-related or modified during secondary processing. The one-year long monitoring of BC was first conducted using a Single Particle Soot Photometer (SP2) from December 2013 to November 2014 in Nanjing, a megacity in the Yangtze River Delta region of China. The seasonal variation in the BC size distribution and mixing state were investigated. There was no apparent systematic variation in the mean BC core mass median diameter between seasons, as these values were 226 +/- 12 nm, 217 +/- 13 nm, 211 +/- 15 nm and 221 +/- 12 nm for winter, spring, summer and autumn respectively. The mixing state of BC was quantified as the bulk relative coating thickness (defined as particle size D(p )over core size D-c, D-p/D-c), which ranged from 1.05 to 2.65. The BC was found to be significantly more coated in the winter (D-p/D-c = 1.50 +/- 0.30) than in other seasons (D-p/D-c = 1.27 +/- 0.09, 1.28 +/- 0.10, 1.27 +/- 0.11 in spring, summer and autumn respectively). Higher levels of coating during the winter may due to the contributions of the primary source (with the highest BC mass loadings between seasons) or secondary processes such as low temperature that facilitated the condensation. It was found that the photochemical process may enhance the coatings on BC in summer. At nighttime, the reduced and stabilized planetary boundary layer and the nighttime secondary formation may also lead to BC becoming well mixed with other components. Moreover. BC was shown to be less coated when the NOx concentration was high. However, during all seasons, the BC coating was strongly correlated with other non-BC particulate mass, which suggests that at higher pollution levels BC was more significantly coated with other existing materials through coagulation or condensation by other secondary species.