The bioconversion of formate to methane was investigated together with microbial community structure in mesophilic sludge enrichment culture fed only with formate using fluorescence in-situ hybridization (FISH), terminal restriction fragment length polymorphism (T-RFLP) and clone libraries.FISH revealed that archaea possessed absolute predominance quantitatively.T-RFLP indicated that key members of archaea with T-RF of 96bp were affiliated with a hydrogen-utilizing methanogen, Methanomicrobiaceae, almost taking up to 90% of the methanogenic consortium.Nevertheless, bacteria owned much higher diversity implied by phylogenetic analyses that 15 selected clones were grouped into three phylotypes (Bacteroidetes, Firmicutes and Synergistetes), but the nearest relatives had no much association with other pathways but archaeal methanogenesis.Consequently, it was not syntrophic formate oxidation or other potential pathways but formates conversion to methane by methanogens that probably be the predominant mechanism in the specific sludge enrichment culture from a molecular microbiological perspective.