The co-digestion of biosolids and organic fraction of municipal solid waste was compared with the direct digestion of biosolids. Addition of organic fraction of municipal solid waste improved carbon-to-nitrogen(C/N) ratio from 8.10 to 17.68 or 20.55 in the feedstock, and led to a higher volatile fatty acid (VFA) concentration of about 1500 mg/L in the solution and a lower solution pH under 6 in the beginning 20-day during digestion. Although the total biogas yield increased by the mixed feedstock, the biogas yield peak was postponed for about 10 days and the maximum biogas yield rate was reduced. After digestion, over 30% of total solid (TS) and over 65% of volatile solid (VS) were removed. However, the COD concentration in the effluent was still higher than 2300 mg/L and needed further treatment before discharge. (c) 2007 Published by Elsevier B.V.