The impact of the threshold concentration of toxic materials on nitrification in stripped gas liquor was investigated. Ammonia nitrogen, phenol, thiocyanide, cyanide, m-cresol, toluene, quionoline, and aniline were selected as toxic materials in the wastewater treatment experiments. The concentrations of organic materials that are contained in raw wastewater of stripped gas liquor were 400 to 600 mg/l phenol, 5.95 mg/l aniline, 17.85 mg/l quinoline, 197.43 mg/l m-cresol, and 85.57 mg/l toluene. When the ammonia nitrogen concentration was lower than 200 mg/l, the nitrification was stable. However, in the case of higher than 200 mg/l in the concentration of ammonia nitrogen, the removal efficiency of nitrogen was very low. Cyanide with concentration higher than 0.5 mg/l acted as a toxic material to microorganisms because it produced excessive foam and made the activity of microorganisms decrease. The threshold concentrations of organic materials such as m-cresol, toluene, quiline, and aniline that influence nitrification of microorganisms were 100 mg/l, 50 mg/l, and 200 mg/l, respectively. The change in the dilution ratio of raw wastewater and the additional amount of PAC did not make a big difference on the COD removal. On the other hand, the higher the dilution ratio of wastewater and additional amount of PAC increases, the higher the removal efficiency of ammonia nitrogen increases.