The objective of this study was to examine the effect of microbial (MO) and fish (FO) oils used in their blends as supplements to the diet containing 80 % of hay and 20 % of barley on fermentation and lipid metabolism in artificial rumen. The three different ratios of n-6 and n-3 fatty acids as the blends of MO and FO were used. The addition of all oil blends did not affect degradation of dry matter (DM), detergent fibre, total gas production, but increased the degradation of cellulose and hemicellulosein the diets. Further, this supplementation significantly increased molar percentage (mol %) of propionate and decreased mol % of acetate. The lipid metabolism in artificial rumen was also affected. The concentrations (mg/g rumen fluid DM) of some isomers of vaccenic and conjugated linoleic acids were also increased by the oil blends. Finally, the oil blends caused the incomplete fatty acids biohydrogenation.