Abstract: The nitrification biofilm was cultivated with the activated sludge from municipal wastewater treatment plant. Under controlled conditions the nitrification biofilm was successfully transitted from nitrification to anaerobic ammonia oxidation (Anammox). During the whole process the number of heterotrophic bacteria decreased, but the number of nitrifying bacteria, denitrifying bacteria and anammox bacteria increased. The increase of three bacterial colonies was supposed to relate to anammox. The result from PCR-DGGE(polymerase chain reaction-denaturing gradient gel electrophoresis) analysis showed that the bacterial colonies obviously changed and the bacterial biodiversity became poorer as the operation time of the anammox bioreactor went on.