A series of in vitro studies were performed to evaluate the effects of unsaturated C18 fatty acid-rich oils and linolenic acid on rumen fermentation and methanogenesis with a high-grain substrate.In oil screening experiment,rapeseed oil,soybean oil and linseed oil were used as tested additives.Rumen fluid from sheep was incubated with ground corn with or without oil supplementation (10% of substrate DM) for 24 h to monitor rumen products.Gas production and methane emission decreased with addition of unsaturated C18 fatty acid-rich oils except rapeseed oil.Soybean oil and linseed oil increased total volatile fatty acids (VFA) production and molar proportion of propionate.In dose-response experiment,the most potent additives——linseed oil,and free linolenic acid were further tested to determine dose-response of rumen fermentation.Linseed oil and free linolenic acid were added to basal substrate at 0%,3%,5%,7% and 9%,respectively.Linseed oil and linolenic acid decreased methane emission significantly and altered rumen fermentation toward more propionate production by supplementation at 5% and 3%,respectively.These results suggested that some unsaturated C18 fatty acid-rich oils and free linolenic acid could be used for improving rumen fermentation under high-grain diet,and the effect of vegetable oils on methane emission was concerned with the type and inclusion levels.In the present study,methane formation was decreased significantly with increasing levels and unsaturation of fatty acids in vegetable oils.