Sludge bulking and its effect on the performance of high-rate denitrifying automatic circulation reactor was studied in this paper. The results showed that, when the nitrogen loading rate and COD loading rate exceeded 65 kg N m(-3) d(-1) and 325 kg COD m(-3) d(-1), respectively, the nitrogen removal rate and COD removal rate decreased from 57.13 kg N m(-3) d(-1) and 232.57 kg COD m(-3) d(-1) to 38.08 kg N m(-3) d(-1) and 174.32 kg COD m(-3) d(-1), respectively. The performance of high-rate reactor deteriorated suddenly due to the insufficient biomass which originated from denitrifying sludge bulking and wash-out. The bulking denitrifying sludge was poor in settlement with the sludge volume index of 150.08 +/- 20.05 mL g(-1), and low in relative settling velocity ratio (average settling velocity to upflow velocity ratio) of 0.90 +/- 0.11. The denitrifying sludge bulking was identified as viscous bulking which was ascribed to the excessive production of extracellular polymers (EPS) (more than 450 mg gVSS(-1)) at the super high loading rate. The viscous bulking can be characterized by a noticeable decrease of bacteria/EPS ratio to 1.22 +/- 0.13 and a large increase of granule water content to 92.46 +/- 0.05%, which indicated that the bulking sludge lost its shape and approached its flow state. (C) 2013 Elsevier B.V. All rights reserved.