The effect of metal oxide nanoparticles (nano-TiO2, nano-Al2O3, nano-SiO2 and nano-ZnO) on anaerobic digestion was investigated by fermentation experiments using waste activated sludge as the substrates. Nano-TiO2, nano-Al2O3 and nano-SiO2 in doses up to 150 milligram per gram total suspended solids (mg/g-TSS) showed no inhibitory effect, whereas nano-ZnO showed inhibitory effect with its dosages increased. The methane generation was the same as that in the control when in the presence of 6 mg/g-TSS of nano-ZnO, however, which decreased respectively to 77.2% and 18.9% of the control at 30 and 150 mg/g-TSS. The released Zn2+ from nano-ZnO was an important reason for its inhibitory effect on methane generation. It was found that higher dosages of nano-ZnO inhibited the steps of sludge hydrolysis, acidification and methanation. Also, the activities of protease, acetate kinase (AK) and coenzyme F-420 were inhibited by higher dosages of nano-ZnO during WAS anaerobic digestion. (C) 2011 Elsevier Ltd. All rights reserved.