In wastewater biological treatment for nitrogen removal, N_2O and NO_x are common gaseous intermediates. Traditional nitrification and denitrification are considered to be the main soueces for them. But with the development of relevant study , more new metablic pathways for nitrogen are found and the production of N_2O and/or NO_x in these processes are experimentally confirmed, which makes the mechanisms for N_2O and NO_x production more complex. Though inhibitory of enzymes and chemical reaction are the probable mechanisms, there are still others unknown. Discussing the sources and mechanisms of N_2O and NO_x in wastewater treatment from the point of contributing microorganisms and biochemistry reaction will benefits the understanding of new nitrogen metabolic pathways, the development of new technology and the protection of air pollution from NO_x.