Abstract: The nitrification in the Anoxic Bio-Sorption Activated Sludge (ABSAS) process for nitrogenremoval was analyzed and discussed.The results show that the effect of organic substances to the nitrification is mainly the competed inhibit of heterotrophic bacteria to nitrification bacteria;the first anoxic reaction could improve the nitrification;this feature could be used in designing nitrogen removal process so as to improve nitrification.