For removing nitrogen from highly nitrogen-containing wastewater, nitrification and denitrification processes applying the sequencing batch reactor (SBR) system and the immobilized cell system were investigated. During the nitrification in SBR process, the time course of NH4+-N concentration exhibited a dramatic and stepwise decrease due to the on/off switch of aeration in the SBR system. Ammonia was converted to nitrate in the aeration stage, and then, it was converted to N2 in the anaerobic stage. However, in the immobilized cell system, the nitrification reaction and the denitrification reaction occurred simultaneously explain the exponential decrease in NH4+-N concentration. In overall, the immobilized cell system is more efficient than the free cell system in wastewater-nitrogen removal.