© 2018 The Society for Biotechnology, Japan Nitrous oxide (N 2 O) is an important greenhouse gas that can be emitted from wastewater treatment plants (WWTPs). Such emissions are reportedly process specific and related to operational parameters. This study was conducted to clarify spatial and daily variations of N 2 O in a full-scale activated sludge anoxic/oxic process that consisted of an anoxic tank and three oxic tanks (oxic-1, oxic-2 and oxic-3), all of which except the final sedimentation tank were fully covered. Higher dissolved N 2 O (D-N 2 O) loading and gaseous N 2 O (G-N 2 O) emissions were observed for oxic-3 than for the anoxic, oxic-1, and oxic-2 tanks, implying that there was higher N 2 O production potential via nitrification in the latter stage of the oxic tank. Moreover, the sudden decrease in dissolved oxygen concentration after the peak was found to lead to abrupt production of D-N 2 O at oxic-3 in the anoxic/oxic process. The increases in AOB amoA, AOB nirK and the following AOB norB gene transcripts at the end of the oxic-2 tank suggested that nitrifier denitrification occurred to produce N 2 O under low dissolved oxygen conditions when the N 2 O peak was observed. Additionally, the much lower transcription levels of the two nosZ genes suggested lower N 2 O consumption. The N 2 O emission factors ranged from 0.087% to 0.302%, and lower N 2 O emission factors were observed during summer.