AbstractTo establish a biotechnological system to convert CO2 into methane, we are trying to develop a new CO2 bio-conversion technology based on “electromethanogenesis”, a new bio-electrolysis reaction using microbially-catalyzed electrode. In this study, we characterized bio-electrochemical properties of electromethanogenic reaction by Methanothermobacter thermautotrophicus strain AH, a thermophilic methanogen. The methanogen can electromethanogenically produce methane without exogenously-supplied hydrogen. In the reaction, the methanogen utilized molecular hydrogen, which was evolved by the abiotic electrochemical reaction, for the hydrogenotrophic methanogenesis. The current-to-methane conversion efficiency was 20% and the hydrogen evolution reaction was the rate-limiting step of the reaction.