This study presents the sludge characteristics and the treatment performances of the SBR in the removal of varying influent phenol concentrations. The SBR was operated by treating synthetic wastewater containing no phenol and with phenol concentrations ranged from 100 to 400 mg/L Operational mode of the SBR included Fill, React, Settle, Draw and Idle periods in the ratio of 2:8:1:0.5:0.5 for a cycle time of 12 h. The sludge characteristics were evaluated through the morphological observation and the sludge settleability while the treatment performance of the SBR was assessed through the phenol removal efficiency. The sludge morphology shifted from predominantly normal floc for SBR treating synthetic wastewater without phenol to predominantly Zoogloeal floc when phenol was introduced to SBR and was increased to 300 mg/L Zoogloeal floc enhanced the sludge settleability. producing clear effluent. As influent phenol concentration increased to 400 mg/L, microfloc was prevailed resulting in poor sludge settleability and deteriorated the quality of effluent with discharged suspended solids. However, the change of sludge morphology did not affect the phenol removal efficiency in the SBR with increase phenol loading. Almost complete phenol removal can be achieved with sufficiently long React mode. (C) 2010 Elsevier B.V. All rights reserved.