The anaerobic ammonium oxidation (ANAMMOX) reactor in an upflow blanket filter (UBF). The authors spent 165 days in successfully transited starting up. The ANAMMOX was from the autotrophic denitrification. After the start-up of the ANAMMOX reactor, the volumetric loading rate was up to 0.17 kg total nitrogen/(m3·d) with the removal rate of 93.04% for NH4+-N and 100% for NO2--N, respectively. When operating at steady state, the molar ratio of NH4--N removal, NO2--N removal and NO3--N production was about 1: 1.18: 0.28. The scanning electron micrographs showed that the dominant bacterium was about 1 μm in size, displayed circular or elliptical morphology, lived in cluster, and had crateriform nick on the surface. The character of the dominant bacterium was the same as that of typical ANAMMOX bacterium.