Simultaneous nitrification and denitrification (SND) for treating coal gasification wastewater (CGW) was achieved successfully in a lab-scale sequencing batch biofilm reactor (SBBR) by oxygen-limited aeration. SND efficiency increased gradually with the concentration of dissolved oxygen (DO) decreased from 4.5 mg/L to 0.35 mg/L. The maximum SND efficiency of 81.23% was obtained at DO concentration of 0.35 mg/L, and the corresponding removal efficiency of NH4+-N and TN reached 76.91% and 70.23%, respectively. Meanwhile, COD was removed significantly and toxic compounds were degraded into biodegradable substances, which relieved effectively the inhibition on nitrogen removal. The results indicated that oxygen-limited condition performed greater toxic compounds and nitrogen removal compared with the aerobic condition. Furthermore, the results of scanning electron microscopic (SEM) and microbial community structure confirmed robust biofilm formation provided a suitable anoxic microenvironment for co-existence of nitrifying and denitrifying bacteria and organics degradation bacteria in the reactor at oxygen-limited condition. (C) 2017 Elsevier Ltd. All rights reserved.