A novel granulation strategy by introducing magnetic nanoparticles (MNPs) into activated sludge system was investigated in this study. The study of the physicochemical characteristics (appearances, sizes, sludge volume index, and chemical oxygen demand) demonstrated that MNPs could decrease the granulation time and improve the retention of biomass, meanwhile enhanced the compact structure of the granules. The secretion and functional groups especially O-H and C=O of extracellular polymeric substances (EPS) also had significant changes under the long-term influence of MNPs. The contents of proteins (PN) and polysaccharides (PS) in R2 (with MNPs) were 95.7523 mg/gVSS and 43.7129 mg/gVSS, while in R1 (without MNPs) they were 85.7523 mg/gVSS and 32.8632 mg/gVSS, respectively. The contact angles of sludge against water dramatically increased with the increase of MNPs concentration, which means that the addition of MNPs could improve the sludge surface hydrophobicity, playing a positive role in the aggregation process. (C) 2016 Elsevier Ltd. All rights reserved.