This paper describes the clarification of the critical organic load and the inhibition factors of thermophilic dry methane cofermentation of municipal solid wastes with various compositions. Various compositions of paper waste per food waste ratio, such as 0.67-0.66, 0.4-0.43, 0.25-0.2, and 0.11, were examined using digesters with an effective volume of 2 L maintained at a thermophilic temperature of 55 °C. It was found that the biogas yield under stable operating conditions was 0.4-0.5 NL/ g VS . The critical organic load of organic matter without fermentation inhibition was 7.5-8.5 gVS/(kg sludge d), and a smaller critical organic load was obtained with a higher food waste input ratio. Because volatile fatty acids (VFAs) accumulated under all conditions when fermentation inhibition occurred, it was considered that fermentation stopped because of the influence of VFAs. Moreover, the propionic acid concentration increased from the time when there was no significant fermentation inhibition, which might be used as a monitoring index for fermentation status.