Abstract: The aim of this study is to review effective factors on decrease methane emission by enteric fermentation, mainly by ruminants. Global surface temperatures are predicted to increase between 1 to 6°C during the twenty-first century; primarily due to increased levels of Greenhouse Gases (GHGs) principally carbon dioxide (CO₂), methane (CH₄) and Nitrous Oxide (N₂O) in the atmosphere. Agricultural emissions of methane have recently been estimated at 10.2 million tonnes per year. Of these, approximately two-thirds come from enteric fermentation and one-third from livestock manure. To discuss factors relation to emissions of GHG’s (specific methane gas) from ruminants, we need to divide them in four groups; nutrition, management, biotechnology and microbiology. In this article, we discussed nutritional factors related to emission of methane gases in ruminants. Other factors (factors related to management, biotechnology and microbiology) will discuss in further articles.