Anaerobic ammonium oxidation (anammox) is a biological process by which ammonium is oxidized to dinitrogen gas by using nitrite as the electrons acceptor. Anaerobic ammonium-oxidizing bacteria play an important role in nitrogen removal from wastewater and global N-cycle. The study of metabolism of anammox bacteria will help us understand the anammox mechanism and develop anammox biotechnology. Anammox bacteria are chemoautotrophic bacteria that use CO, or HCO3- as carbon source and obtain their energy from the conversion of ammonium and nitrite into dinitrogen gas. Hydrazine has been detected as an intermediate in the anammox pathway, while hydroxylamine and nitric oxide have not been detected yet. The genomic data indicate that anammox bacteria fix carbon dioxide through acetyl-CoA pathway. The proposed anammox pathway is consistent with the available experimental data, thermodynamical calculation and biochemical determination and as well as the Ockham's razor principal.