WRF-Chem was used to study a severe haze episode that occurred over the Yangtze River Delta (YRD), China, in November 2013. This episode was characterized by a high PM2.5 concentration (> 400 µg m–3), high relative humidity (> 80%) and low visibility (< 900 m). Regional average results showed that PM2.5 concentration peaks corresponded closely with a low wind speed and a low planetary boundary layer (PBL) height, and the maximal PM2.5/PM10 ratio of 0.89 indicated fine particle dominance. Horizontal dispersion analysis showed that the ventilation coefficient (VC) dropped from above 3000 m2 s–1 (clean days) to below 1500 m2 s–1 (polluted days), and the average VC for December for the period of 2008–2012 was 2119 m2 s–1; horizontal transport flux showed central and northwest YRD mainly outputted pollutants in this episode. Vertically, because of the influence of the PBL and nocturnal inversion, the region of high PM2.5 concentration (> 125 µg m–3) extended to 1 km height during daytime, but was confined to below 200 m at night. However, near-surface inversion was observed even on clean days. Therefore, we concluded that poor horizontal dispersion ability played a dominant role in the haze formation, and weak vertical dispersion ability, together with high relative humidity, aggravated the pollution. Chemical analysis showed that, compared with PM2.5 in the northwest part of the YRD, PM2.5 in the central YRD contained a higher proportion of nitrate and a lower proportion of black carbon and organic carbon. The observed NO3–/SO42– ratio was 1.54 for this episode, and the monthly average dropped to 1.40 for December 2013. We concluded that mobile sources contributed considerably to the episode. Moreover, in winter, higher NO3–/SO42– and nitrate being the main component (29%) in PM2.5 made central YRD different from the Beijing-Tianjin-Hebei region.