The effect of organic loading rate (OLR) on the granulation process was evaluated using upflow anaerobic sludge blanket (UASB) reactors running under different conditions. Results showed that increase of OLR, extracellular polymer (ECP) content and granulation were closely related to one another. ECP in the sludge accumulated over a short period under overloading conditions, which greatly enhanced the granulation process. Treatment performance could be recovered after the ECP accumulation when the overloading was suitably exerted. However, too high loading rate should be avoided because it could cause the unrecoverable decay of methanogenic activity and the serious unbalance between the feedstuff and biological requirement. (c) 2006 Elsevier Ltd. All rights reserved.