This study was conducted to determine the optimal substrate/inoculum (S/I) ratio for solid state anaerobic digestion (SS–AD) of beef manure and sawdust bedding mixtures. The organic material removal and methane yield performance were evaluated at five different S/I levels (1, 2, 4, 50, and beef manure alone on a VS basis) for 49 days at 37 C. Results showed that biodegradable volatile solids (BVS) reduction of beef manure alone test unit was 76% which was not significantly different from the rest of all inoculated test units. The methane production potential of all five test units showed similar levels (135 to 154 mL/g VS), regardless of S/I ratio. Although it showed similar methane production potential, the beef manure alone and S/I ratio 50 test units showed the highest methane yields of 7.3 and 7.2 based on the volume of methane produced per volume of digester, respectively. In addition, when considering utilization efficiency of the available digester volume, the beef manure alone treatment was 15% higher than that of the S/I ratio 50 treatment. These results reveal that the beef manure and sawdust bedding mixture does not require inoculation to improve SS–AD performance.