Complete nitrification was investigated with synthetic wastewater containing high ammonium nitrogen concentration and low COD by the activated sludge as a chemo-autotrophic process in the Continuously Stirred Tank Reactor (CSTR). The nitrification process was operated under the optimal conditions to achieve the complete nitrification. Then inhibition of complete nitrification process by adding aniline step wise was studied. It was found that nitrification process started to be inhibited at aniline concentration of 20 mg/L. Ammonium removal efficiency and nitrate formation efficiency were swiftly plunged, especially inhibition of aniline on nitrite oxidation was more than on ammonia oxidation. Fluorescence in situ hybridization (FISH) technique was also used to study the nitrifier population shift for completed and inhibited nitrification. Results shown that inhibitor aniline affected to nitrifying microorganism population. And aniline affected to the reduction of nitrite oxidation bacteria population was more than that of ammonium oxidation bacteria.