Abstract: A new upper boundary of gas hydrate zone in marine sediment is defined by this paper. ODP data show that the amount of biomass in the marine sediments is among 70% of the total global prokaryota, and its carbon content is comparable to carbon content of terrestrial plants. So the huge microbial life in the marine sediment must play a very important role in the geological processes like methane gas distribution in the earth crusts. Methane is widely distributed within the earth's crust and mainly the source is from bacterial or biogenic rather than thermogenic origin. There are two ways for the microorganism to produce methane, one is from carbon dioxide reduction and one is from acetate fermentation, and accordingly, the methane producing bacteria is mainly methanogenic archaea and acetotrophic methanogen. Anaerobic methane oxidation in marine sediment environment has been confirmed and accepted for a time. This procedure happens within a very limited zone namely sulfate reduction and methane anaerobic oxidation zone and usually sulfate-methane interface. This is a basic biogeochemical interface within the marine sediment. It largely limits the release of methane from marine sediment up to the seafloor. The anaerobic methane oxidation is also a microbial mediated procedure, dominated by methanotrophic archaea and sulfate reducing bacteria. The sulfate-methane interface in marine sediment can be more than one hundred meter deep below seafloor, however, sometime it can be up near to the seafloor. This interface serves as the upper boundary of gas hydrate zone in marine sediment,and there will be no gas hydrate above this interface.