Abstract: The effects of polyunsaturated fatty acid from soybean oil and cottenseed oil on the accumulation of Trans vaccenic acid (TVA) -biohydrogenation of ruminal microbes and rumen fermentation were researched with artificial rumen.Results indicated that TVA was accumulated with treatments of soybean oil and cottenseed oil, especially 4% treatments were abstractly higher than control(P<0.05).The concentrations of total volatile fatty acid (TVFA) of all treatments were higher than control, cottenseed oil treatments were obviously higher than control(P<0.05).The concentrations of NH₃-N were also higher than control,treatments of 2%soybean oil ,2%cottenseed oil and 4%cottenseed oil were obviously higher than control(P<0.05).