Long-term performance of a lab-scale UASB reactor treating starch wastewater was investigated under different hydraulic retention times (HRT). Successful start-up could be achieved after 15 days' operation. The optimal HRT was 6 h with organic loading rate (OLR) 4 g COD/L d at COD concentration 1000 mg/L, attaining 81.1-98.7% total COD removal with methane production rate of 0.33 L CH4/g CODremoved. Specific methane activity tests demonstrated that methane formation via H-2-CO2 and acetate were the principal degradation pathways. Vertical characterizations revealed that main reactions including starch hydrolysis, acidification and methanogenesis occurred at the lower part of reactor ("main reaction zone''); comparatively, at the up converting acetate into methane predominated ("substrate-shortage zone''). Further reducing HRT to 3 h caused volatile fatty acids accumulation, sludge floating and performance deterioration. Sludge floating was ascribed to the excess polysaccharides in extracellular polymeric substances (EPS). More efforts are required to overcome sludge floating-related issues. (C) 2015 Elsevier Ltd. All rights reserved.