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The aim of the thesis was to evaluate the effect of probiotics on ration digestibility, rumen parameters in vitro, productivity and production quality of dairy cows. For this study 20 dairy cows were selected and using the principle of analogues divided into control and trial groups (10 cows per group). Studies lasted for 90 days. During the study dairy cows were fed twice a day after milking. Both groups received identical diet which was balanced based on the animals’ needs. Trial group ration was supplemented with 20 g per cow of probiotic feed additive Biogrom SC, which contained Saccharomyces cerevisiae (CNCM I-1077 5x1011 cfu/kg). Milk yields were recorded and milk samples from all experimental cows were collected during control milking at the beginning, middle and the end of the study. The effect of probiotic feed additive on ration digestibility, rumen parameters and ruminal gas formation were evaluated using in vitro RUSITEC system. It was determined that probiotic feed additive had a significant effect on increasing rumen pH. Other rumen parameters and ruminal gas formation were not significantly affected, however, a tendency to have decreased amount of ruminal nitrogen and increased amount of volatile fatty acids was monitored. Probiotic feed additive had no effect on the trial groups ration dry matter, crude protein and crude fiber digestibility in rumen, but non-azotic extractable matter digestibility was lower in the trial group than it was in the control group. However, a tendency to increased crude fat and crude ash digestibility was monitored. The results show that feed additive had no significant effect on milk yield, but milk fat and milk lactose increased significantly.