The rumen simulating technique (RUSITEC) was used to evaluate the effect of 3 diets with different ratios of conserved feeds on rumen fermentation and the concentration of fatty acids (FA) in fermentation fluid. Diets included 30 % of alfalfa silage and 70 % of corn silage (KD1), 30 % of alfalfa silage, 38 % of corn silage and 32 % of grain-legume silage (KD2) or 20 % of corn silage and 80 % of grain-legume silage (KD3), respectively, on dry matter basis (DM). We observed the lowest values of DM (P0.01) and crude protein (CP) degradability (P0.001) in diet KD3 and acid-detergent fiber (ADF) degradability in diet KD1 (P0.001). The degradability of neutral-detergent fiber (NDF) wasn´t influenced by composition of diets. The highest production of ammonia nitrogen (NH3-N) (mg/l) in effluent was analysed in vesels with KD1 (P0.001), however the highest microbial activity in nitrogen synthesis was observed in vessels with KD3 (P0.001). Production of fermentation gasses, methane and volatile fatty acids wasn´t influenced. Content of saturated FA (SFA), mono- (MUFA) or polyunsaturated FA (PUFA) wasn´t alter depending on composition of diets – the diet KD3 had the highest contents of trans-vaccenic (P0.05) and oleic FA (P0.001) and 10,12 – isomer of CLA (P0.01), resp.