The impact of ozonation on physicochemical properties of sewage sludge and the relationships between the physicochemical properties and dewaterability during ozonation were investigated. CST was used to evaluate sludge dewaterability. The results showed that the settleability of sludge was improved after ozonation. The relative hydrophobicity (RH) of sludge flocs decreased from 69.2 +/- 4.0% to 50.0 +/- 2.4% after ozonation. The dewaterability of sludge was affected by ozonation and depended on ozone dose as CST increased from 25.5 +/- 2.4 s to 289.0 +/- 25.9 s during ozonation. The SCOD and DDSCOD increased significantly after ozonation, but the increasing tendency of DDSCOD became slow when ozone dose was above 37.8 mg O-3/g SS. The concentrations of polysaccharides and proteins in supernatant increased from 4.46 +/- 0.21 mg/L to 220.90 +/- 24.87 mg/L and from 6.26 +/- 0.28 mg/L to 386.54 +/- 32.15 mg/L, respectively. The ratio of proteins and polysaccharides in supernatant increased from 1.40 +/- 0.06 to 1.75 +/- 0.08 after ozonation. The DDSCOD and constitution of EPS in supernatant played a very important role in changing the dewatering performance of sludge. Microscope images and BET surface area demonstrated that sludge was disintegrated and microbial cells were destroyed by ozonation treatment. The mechanism involved matter transformations during ozonation was also discussed. (C) 2016 Elsevier B.V. All rights reserved.