A greater understanding of anaerobic ammonium oxidation bacteria will help to pave the way for the new biological nitrogen removal process application in practice. To this end, this study used molecule biology methods. Crude DNA of the total bacteria in a cultivated sludge with ANAMMOX capability was extracted and purified. Then PCR amplification using specific primers, clone and sequencing processes were performed. The partial 16S rDNA sequence of cultivated ANAMMOX bacteria is 836bp. Some clones have one to two base mutation(s). Phylogenetic analysis indicates that the cultivated ANAMMOX bacteria in this study close to Candidatus Brocadia anammoxidans, anaerobic ammonium-oxidizing Planctomycete and uncultured anoxic sludge bacterium KUI with the same function, whereas the cultivated ANAMMOX bacteria are relatively low DNA sequence similarity to the forementioned bacteria using alignment analysis. The results suggest that there is a kind of bacterium which has never been found before with ANAMMOX capability existing in nature.