The objective of this study was to evaluate the nitrous oxide (N2O) emissions in partial nitrifying and full nitrifying granular sludge reactors treating ammonium-rich wastewater. During stable operation, there was no significant difference of NH4+-N removal efficiencies between the two granular reactors. Nitrate and nitrite were the main effluent nitrogen species of the two reactors, and nitrite accumulation rate of partial nitrifying reactor was high of 87.79 +/- 2.03%. However, partial nitrifying granular-reactor had better total nitrogen removal efficiency (41.84 +/- 3.35%) than that of full nitrifying granular-reactor (19.91 +/- 2.12%). According to typical cycles, the N2O emission amount per cycle of partial nitrifying reactor account for 11.48% of the incoming nitrogen load, which was 1.5 times higher than that of full nitrifying reactor (7.47%). The obtained results could provide more information for understanding of N2O emission in granular sludge systems. (C) 2014 Published by Elsevier Ltd.