Selective nitrifications of high strength ammonia wastewater was performed with biofilm airlift suspension reactor for the economical removal of nitrogen via nitrite. Biofilms were successfully attached on the surface of the carriers(sand) and the biomass concentration reached up to 15 g/L with the reactor. During the experiment more than 90% nitrification was obtained at 2.5 kg NH4 +-N/m(3)/d and selective nitrification was successfully carried out to have more than 90% nitrite accumulation of the total oxidized nitrogen. Nitrite accumulation was achieved by the inhibitions on nitrite oxidizers with free ammonia and low dissolved oxygen in the biofilm airlift suspension reactor. From the batch nitrification analysis it was shown that the existence of free ammonia is the main cause of nitrite accumulation.