The research was conducted to investigate the microbial diversity and population with the different concentration of NH4+-N in a biofilm reactor filled with volcanic filter for completely autotrophic nitrogen-removal over nitrite (CANON) process. The reactor had an excellent performance with the decreasing of NH4+-N concentration from 400 to 200 mg L-1 while NH4+-N removal loading reduced at the NH4+-N concentration of 100 mg L-1. Biodiversity analysis indicated that Nitrosomonas related aerobic ammonia oxidizing bacteria (AOB) and Planctomycetales-like anaerobic ammonia oxidizing (anammox) bacteria were dominant functional bacteria. Despite the different influent NH4+-N concentration, anammox bacteria had a low and stable biodiversity, which was not the same to AOB. With the concentration reduction of influent NH4+-N, the estimates of total bacteria population ranged between 2.29 x 10(11) and 1.44 x 10(12) copies mg(-1) total DNA, and the quantity of AOB decreased while anammox bacteria kept stable. The population of Nitrospira increased and little Nitrobacter was detected during the experiment. (c) 2012 Elsevier Ltd. All rights reserved.