A new method for the conversion of CO to H<SUB>2</SUB> was developed by anaerobic mixed culture in the current study. Higher CO consumption rate was obtained by anaerobic granular sludge (AGS) compared to waste activated sludge (WAS) at 55<SUP>o</SUP>C and pH 7.5. However, H<SUB>2</SUB> was the intermediate and CH<SUB>4</SUB> was the final product. Fermentation at pH 5.5 by AGS inhibited CH<SUB>4</SUB> production, while the lower CO consumption rate (50% of that at pH 7.5) and the production of acetate were found. Fermentation at pH 7.5 with the addition of chloroform achieved efficient and selective conversion of CO to H<SUB>2</SUB>. Stable and efficient H<SUB>2</SUB> production was achieved in a continuous reactor inoculated with AGS, and gas recirculation was crucial to increase the CO conversion efficiency. Microbial community analysis showed that high abundance (44%) of unclassified sequences and low relative abundance (1%) of known CO-utilizing bacteria Desulfotomaculum were enriched in the reactor.