Extracellular polymeric substances (EPS) was believed to be the primary factor determining the sludge dewaterability due to its ability to bind with water. Conventionally, several different mechanisms were proposed for the enhanced dewaterability. This study firstly clarified the correlation between EPS structure/property changes and improved sludge dewaterability. The characteristics of both extracted EPS from waste activated sludge and the sludge itself before and after the treatment with the Fe(II) activated peroxidation process, i.e. Fenton's conditioning, were investigated. The treatment with Fenton's reagent improved sludge dewaterability, with the EPS structure changed as well. It was found that both EPS and cells were solubilized by comparing the release of protein and polysaccharide from extracted EPS and sludge itself after peroxidation. The increased dewaterability was thus likely achieved through the destruction of both EPS (including loosely-bound and tightly-bound EPS) and cells by Fenton's conditioning while other mechanisms (i.e. flocculation/oxidization) played a secondary role.