A batch anaerobic test was conducted to evaluate the effects of inoculum/substrate ratios (ISRs) on the methane yield and orthophosphate release from the anaerobic digestion of Microcystis spp. The results demonstrated an obvious influence on methane yield and orthophosphate release by ISR. The maximum methane yield decreased from 140.48 to 94.42 mL/g VS when the ISR decreased from 2.0 to 0.5. The highest maximum methane yield calculated from Orskov equation was 153.66 mL/g VS at ISR value of 1.0. The values of pH, ammonia and volatile fatty acids (VFAs) corroborated the appropriate stability of this anaerobic process. (C) 2010 Elsevier B.V. All rights reserved.