Abstract: In vitro batch culture techniques were used to investigate the effect of S. ruminantium alone and in combination with yeast cultures on rumen bacterial fermentation. When Leymus chinensis was used as a substrate, yeast culture addition increased the acetate, propionate, butyrate, valeric acid, isovaleric acid and TVFA contents (P<0.05), and reduced lactic acid content, but had no significant effects on pH value, isobutyrate or ratio of acetate to propionate (P<0.05). The ratio of acetate to propionate and the lactate concentration were reduced (P<0.05), and acetate, propionate, n-butyrate, isobutyrate, isovaleric acid, TVFA content and pH value were improved by S. ruminantium supplementation (P<0.05). There was no significant interaction between pH value, acetate, propionate, butyrate, isovaleric acid or TVFA content between S. ruminantium and yeast cultures (P>0.10). When maize was used as a substrate, live-yeast culture addition increased pH value, acetate, propionate, n-butyrate, isobutyrate, isovaleric acid and TVFA content (P<0.05), and reduced lactate concentration and the ratio of acetate to propionate (P<0.05), but there was no significant effect on isobutyrate content (P>0.10). pH value, acetate, propionate, butyrate, isovaleric acid and TVFA content were improved by S. ruminantium, and S. ruminantium supplements lowered lactate content and the ratio of acetate to propionate (P<0.05). There was a significant interaction effect on pH value, lactate, acetate, propionate butyrate, TVFA content and the ratio of acetate to propionate between S. ruminantium L9 and yeast cultures (P<0.05). When starch was used as a substrate, S. ruminantium had an increase in pH value and acetate, propionate, butyrate and TVFA content (P<0.05), but a decrease in lactate concentration and the ratio of acetate to propionate (P<0.05). Yeast culture supplement increased pH value and acetate, propionate, butyrate and TVFA content (P<0.05), but had no effect on valeric acid, isobutyrate content or the ratio of acetate to propionate (P>0.10). A significant interaction effect was observed on pH value, acetate, propionate, butyrate, TVFA, lactate content and the ratio of acetate to propionate between S. ruminantium and yeast cultures (P<0.05). S. cerevisiae L9 can be considered a good strain which might tolerate the adverse conditions in the gastrointestinal tract when used as a live microbial feed supplement in the diet of the animals. Compared with the addition of yeast or S. cerevisiae alone, the live-yeast cultures combined with S. cerevisiae L9 more effectively to reduce lactate and maintain a stable ruminal pH.