A denitrifying/nitrifying membrane biofilm reactor for simultaneous removal of Hg-0 and NO was investigated. Hg-0 and NO removal efficiency attained 94.5% and 86%, respectively. The mercury-oxidizing microbial community was significantly shaped by nitrification/denitrification after the supply of gaseous Hg-0 and NO continuously. Dominant genera Rhodanobacter and Nitrosomonas participated in Hg-0 oxidation, nitrification and denitrification simultaneously. Hg-0 oxidizing bacteria (Gallionella, Rhodanobacter, Ottowia, Nitrosomonas and etc.), nitrifying bacteria (Nitrosomonas, Rhodanobacter, Diaphorobacte and etc.) and denitrifying bacteria (Nitrosomonas, Rhodanobacter, Castellaniella and etc.) co-existed in the MBfR, as shown by metagenomic sequencing. X-ray photoelectron spectroscopy (XPS) and high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) confirmed the formation of a mercuric species (Hg2+) from mercury bio-oxidation. Mechanism of mercury oxidation can be described as the bacterial oxidation of Hg-0 in which Hg-0 serves as electron donor, NO serves as electron donor in nitrification and electron acceptor in denitrification, oxygen serves as electron acceptor.