With the development of sequencing and in vitro culture technique, more and more unknown methanogens have been identified. In recent years, the 7^th order of methanogen (Methanomassiliicoccales, Mmc) has been gradually discovered and established. Mmc is close to Thermoplasmatales in evolutionary ralationship, but it exists at a relatively long distance and forms an independent cluster. Mmc is widely distributed in the digestive tract of mammalian and insects, paddy fields, wetlands and other environments, but strains from different environments exhibit habitat preference. Mmc lacks the complete metabolic pathway to reduce CO₂ to methyl coenzyme M, which leads to their strict use of hydrogen to reduce methyl substrates to genetrate methane. A comprehensive and in-depth understanding of the functions of Mmc in rumen of ruminants will be conducive to the proposal of a new and efficient strategy for methane emission reduction in ruminants. Therefore, we review here the isolation culture, physiochemical and genomic properties of Mmc as well as its role in the rumen methane production.