Methane is a greenhouse gas formed mainly from the microbial degradation of feeds in the ruminant digestive tract. Methane emissions from ruminants not only result in the loss of a large amount of feed energy but also contribute to the global warming. Forage brassicas are important temporary fill-incrops with high nutritive values. When fed to ruminants,methane emissions largely decrease. The mitigation effects can exist in different seasons under grazing or housing conditions when fed alone or as a part of the diet. Thus,forage brassicas could be an approach to the mitigation of methane emissions with a great potential to use in practice. To find out the possible causes for the low emissions with forage brassicas,feed chemical composition,rumen fermentation parameters and ruminal microbes were explored,but the mechanisms are still not identified. This review summarized effects of forage brassicas on mitigation of methane emissions from ruminants and proposed possible mechanisms.