This study was conducted to examine the effects of yeast culture on rumen fermentation when fed to dairy cows directly. Four ruminally-fistulated Holstein cows were used in 4×4 Latin square design. Each cow was directly fed one of the diets (50 g/d): no addition (the control), Sacchromycopsis fibuligera (SF), Saccharomyces cerevisiae (SC), and Candida tropicalis Berkhout (CB). The whole trial lasted for 18 d with 15 d of adaptation period and 3 d of sampling period. The results showed as follows: dietary yeast culture could significantly increase rumen pH (P<0.05) at 4-8 h, with CB yeast culture being the best, which could increase pH from 6.24 to 6.45-6.59. Dietary yeast culture also significantly increased the concentration of ammonia nitrogen by 11.64%-16.09% (P<0.05) at 4 h. The concentrations of total ruminal volatile fatty acids, acetic acid, butyric acid, and acetate/propionate ratio were significantly increased (P<0.05) when cows fed yeast culture at 6-8 h. Total number of rumen bacteria and cellulase activity were significantly increased by 25.00%-59.25% and 6.13%-11.83% at 4-8 h, respectively (P<0.05), and rumen protozoa number was not altered by treatments (P>0.05). All indices were not different among 3 treatments (P>0.05). In conclusion, yeast culture affects rumen fermentation and consequently improves feed utilization.［Chinese Journal of Animal Nutrition, 2010，22（5）:1390-1395］