Abstract: The inhibitory effect of octylphenol and related main factors on activated sludge was investigated by measuring the specific oxygen uptake rate of activated sludge. The results were as follows. (1) There was a slight inhibitory effect on activated sludge when octylphenol concentration was 8 mg/L. The effect was strengthened with the increase of the concentration of octylphenol, and the leveling off of this change when octylphenol concentration reached 64 mg/L. (2) The EC50 =38.6 mg/L. (3) Sludge concentration and reaction time were the main factors for the inhibition of activated sludge. The inhibitory effect was strengthened with the increase of reaction time and the decrease of sludge concentration. (4) The activity of inhibited sludge will be recovered partially when the octylphenol was removed from experimental system.