The correlation between sludge reduction induced by 2,4,6-trichlorophenol (2,4,6-TCP) as an uncoupler and sludge toxicity was investigated in sequence batch reactors over a 100-d operation period. The influent concentrations of 2,4,6-TCP tested were 10 mg/L, 30 mg/L, and 50 mg/L. Sludge reduction, chemical oxygen demand (COD) removal rate, and sludge toxicity were measured. The results showed that from 30 to 80 d, when the COD removal rate was at an acceptable level, the sludge reduction levels for the 10 mg/L, 30 mg/L, and 50 mg/L groups were 9.7%, 31.6%, and 41.5%, respectively, and the average sludge toxicity values were 24.2%, 38.0%, and 53.0%, respectively. Sludge reduction was positively correlated with sludge toxicity. The two-dimensional polyacrylamide gel electrophoresis/results showed that extracellular and intracellular proteins secreted by the activated sludge during uncoupling metabolism were positively correlated with sludge toxicity. Taking the COD removal rate, sludge reduction, and sludge toxicity into consideration, the optimal influent concentration of the uncoupler 2,4,6-TCP was 30 mg/L when the initial mixed liquid suspended solids of sludge was 2,500 mg/L.