In this study, effects of reduced graphene oxide (RGO) on the activities of anammox biomass and key enzymes were investigated, which was the first time to relate the enzymes activities and RGO together. The batch experimental results demonstrated that the optimal dose grapheme oxide (GO) with 100 mg/L could enhance the total nitrogen (TN) removal rate of 17.2%. Besides, the hydrazine dehydrogenase (HDH), nitrite reductase (NIR), and nitrate reductase (NAR) activities were enhanced with different dose RGO addition, about 0.70-2.75 folds compared to the control experiments. Moreover, the restoration of HDH activity with exogenous coenzyme Q (CoQ) and RGO addition proved that RGO was much more efficient than exogenous CoQ to recover the HDH activity in the extracted CoQ system. Therefore, it could infer that RGO had much better electron transfer ability compared to CoQ, which would be the root reason for the effects of the RGO on anammox process. (C) 2015 Elsevier B.V. All rights reserved.