Abstract A renovated suspended carrier biofilm reactor was investigated for its capability of carbon oxidation and nitrification. The carrier used was polyvinyl chloride cylinder with 2.5 mm diameter and 2.5–3.0 mm height. The carrier filling ratio, which is defined as the volume ratio of carrier to the whole reactor, is a key characteristic parameter for the new reactor. The operational experiments under different carrier filling ratio ranging from 10 to 75% were conducted. Meanwhile, the pollutant removal rate and behaviour of the microorganisms, such as biomass and specific oxygen utilization rate (SOUR) activity, were monitored and analyzed. The results showed that when the influent chemical oxygen demand (COD) and ammonia was about 200 and 20 mg/l, hydraulic retention time of 1.0 h, the optimum carrier concentration was about 50%with the average COD and ammonia removal rate about 70 and 30%, respectively.