Simultaneous nitrification and denitrification (SND) was evidently observed in a full-scale multi-channel oxidation ditch (OD), especially occurring within the outer channel under microaerophilic conditions. The optimum average dissolved oxygen (DO) concentration for SND was 0.25 mg L-1 with 79% SND efficiency and 5.09 mg L-1 h(-1) SND rate, leading to 74.9% overall TN removal efficiency. Small sized flocs consisting of more nitrifying bacteria enhanced nitrification in favor of SND due to less oxygen diffusion limitation inside flocs than larger flocs under micro-aerated conditions. SND was also identified by co-existing of nitrifying and denitrifying bacteria as well as some microaerophilic microorganisms based on molecular biology analysis. Nitrogen mass balances (NMB) quantitatively demonstrated that the highest SND efficiency accounted for 44.1% of influent nitrogen mass. Above valuable findings might deeply reveal SND as a principal mechanism of nitrogen removal in a multi-channel OD from macro- to micro-scale. (C) 2014 Elsevier B.V. All rights reserved.