The objective of this study is to determine whether an autoclaved yeast culture of Saccharomyces cerevisiae or live yeast has an effect on the parameters of ruminal fermentation. For this purpose, autoclaved yeast cultures and live yeast were tested simultaneously by the rumen simulation technique (Rusitec). Each fermentation vessel received daily  15 g feed bag consists of 7.5 g dried basal feed (5 g grass silage + 2.5 g corn silage) and 7.5 g concentrate pellets. Two experiments with two fermentation vessels were carried out. In exp 1 the 1^st vessel (control) received 7.5 g energy concentrate in addition to 7.5 g dried basal feed. The second fermentation vessel (T1) received 5 g energy concentrate plus 2.5 g protein concentrate containing autoclaved yeast culture in addition 7.5 g basal feed. In exp 2 the vessel 1 received the same control diet as in experiment one, while vessel 2 (T2) received 5 g energy concentrate + 2.5 g of protein concentrate containing autoclaved yeast culture and live yeast in addition to 7.5 g basal feed. Yeast culture influenced the ruminal fermentation kinetics in terms of lower (P<0.05) gas volume and higher acetate (mol %), with a concomitant decrease in iso-valerate (mol %) in experiment 1, moreover, yeast culture numerically not statistically increase individual and total volatile fatty acids. in experiment 2, the living yeast cells decreased gas volume more than yeast culture but not alter any other rumen fermentation kinetics. In conclusion autoclaved yeast culture and living yeast cells had beneficial effects on rumen fermentation and this effect was more pronounced with live yeast culture.