The characteristics of the long term (90 d) anaerobic semi-continuous digestion of food wastes/animal slurry wastewater with different C/N ratio were investigated. The research demonstrates that, in the semi-continuous digestion process, different C/N ratios of substrate led to a significant effect to the characteristic of anaerobic digestion (AD). It is undoubtedly that the semi-continuous AD is efficient within a wide range of C/N ratios, and the lower C/N substrate promoted the activity of the methanogen in a long time, which maintained a stable high CH4 concentration and total organic carbon (TOC) utilization. During the whole AD process, an active buffer system was formed from NH4+ and VFAs (by-products of TOC): this buffer system provided high concentrations of VFAs, thus increasing both the CH4 yield and TOC utilization; and also maintained a high tolerance to NH4+ and VFAs in the system, which defusing the impact of NH4+. (C) 2013 Elsevier Ltd. All rights reserved.