To enhance biogas production and identify new additive materials for the co-digestion of wheat straw, sheep manure, and chicken manure, batch experiments were investigated in this study. Experiments were conducted on the influence of additive materials on a range of manure/straw ratios (3:7, 5:5, and 7:3) and biogas production under a mesophilic temperature (35 degrees C). Results showed that the maximum increments of methane production (L/kg . VSadd) with the addition of 3 g magnetite and 1 g natural zeolite were 52.01% and 51.01%, respectively. The addition of magnetite and zeolite in the anaerobic digestion process produced a good fermentation environment. By using the response optimizer when the manure proportion was 52%, the best methane yield was obtained with the addition of 2.7g magnetite. For zeolite, the best addition dose was I g and the optimum manure proportion is 63%. Magnetite had a more extensive increase in methane yield than zeolite. (C) 2015 Elsevier Ltd. All rights reserved.