Abstract: In order to study the characteristics of microbe and organic nitrogen transformation along the reactor of Up-flow Biological Aerated Filter(BAF) and optimize the design and work of the filter with the sewage containing organic nitrogen. Five sampling ports should be set along the reactor. And at 0.329～0.505 m3/(m2?h) of hydraulic loading, (4～5)︰1 of the air/water ratio, 3.0～5.8 mg/L along the reactor of DO with no organic carbon. The results showed that simultaneous nitrification and ammonization were the major process in the reactor. More than 90% DON was transformed into NO3--N. The ammonization rate was obviously faster than nitrification rate among 0～0.29 of the relative attitude of the filter, however, both nitrification and ammonization rate were declined and tend to balance finally as the height of filter media raised. There was few NO2--N left along reactor. It was found that the biomass of each segment decreased from the bottom to the top of the reactor. Each segment along reactor could form steady micro-ecosystem. A decreasing trend of OUR was observed at the order of 101 mgO2/(g?h) in the BAF reactor. The study can provide theoretic basis for the design of the reactor and technique for the optimization of working parameters.