Simultaneous nitrification denitrification (SND) process was achieved in a SBR system to evaluate the impacts of intracellular carbon source PHB and oxygen uptake rate (OUR) on nitrous oxide (N2O) emission. Compared with the sequential nitrification and denitrification (SQND) process, SND process significantly improved the nitrogen removal. N2O emission during SND process was much higher than the SQND process. The amount of N2O emission was 26.85 mg N per cycle in SND process, which was almost four times higher than that in SQND process. About 7.05% of the removed nitrogen during SND process was converted to N2O-N. N2O emission had great relations with the OUR and the OUR could reflect the N2O emission trend more exactly than the DO concentration. At the aerobic stage of SND, the simultaneous denitrification could carried out using PHB as the carbon source and N2O emission increased because of the slow degradation of PHB. (C) 2011 Elsevier Ltd. All rights reserved.