Emission of nitrous oxide (N 2 O) during biological wastewater treatment is of growing concern since N 2 O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N 2 O from a lab-scale granular sequencing batch reactor (SBR) for partial nitrification (PN) treating synthetic wastewater without organic carbon was therefore determined in this study, because PN process is known to produce more N 2 O than conventional nitrification processes. The average N 2 O emission rate from the SBR was 0.32 ± 0.17 mg-N L -1 h -1 , corresponding to the average emission of N 2 O of 0.8 ± 0.4% of the incoming nitrogen load (1.5 ± 0.8% of the converted NH 4 + ). Analysis of dynamic concentration profiles during one cycle of the SBR operation demonstrated that N 2 O concentration in off-gas was the highest just after starting aeration whereas N 2 O concentration in effluent was gradually increased in the initial 40 min of the aeration period and was decreased thereafter. Isotopomer analysis was conducted to identify the main N 2 O production pathway in the reactor during one cycle. The hydroxylamine (NH 2 OH) oxidation pathway accounted for 65% of the total N 2 O production in the initial phase during one cycle, whereas contribution of the NO 2 -reduction pathway to N 2 O production was comparable with that of the NH 2 OH oxidation pathway in the latter phase. In addition, 3 spatial distributions of bacteria and their activities in single microbial granules taken from the reactor were determined with microsensors and by in situ hybridization. Partial nitrification occurred mainly in the oxic surface layer of the granules and ammonia-oxidizing bacteria were abundant in this layer. N 2 O production was also found mainly in the oxic surface layer. Based on these results, although N 2 O was produced mainly via NH 2 OH oxidation pathway in the autotrophic partial nitrification reactor, N 2 O production mechanisms were complex and could involve multiple N 2 O production pathways.