The experiment was carried out on a Rusitec (rumen simulation technique). The effect of diet (fresh lucerne plus maize, 40:60%) and fat sources (linseed LO, rapeseed RO, fish FO oils, 5% wt·wt^-1) on rumen fermentation was studied. To ensure a steady-state within the Rusitec vessels a 7-day adaptation period preceded the 6-day collection period. The diet and a 5% addition of LO, RO or FO were added to the respective fermentation vessels daily. The results showed that oil supplementation (LO, RO, FO) did not affect some basal parameters of rumen fermentation (pH, total VFA production, dry matter and detergent fibre digestibility), but significantly reduced the degradation of cellulose. Among the oils, mainly LO significantly reduced mol% of acetate, increased the mol% of propionate, however, all three oils significantly reduced the A/P ratio. The oils supplemented to the high concentrate diet decreased (P < 0.001) the efficiency of microbial protein synthesis.