A metabolism trial was conducted with three ruminally fistulated sheep in a 3 × 3 Latin square design to investigate the effect of feeding yeast diets on fermentation characteristics, viable mixed bacterial number, degradation of the diets in the rumen, whole tract digestibility and N retention. Following three diets were fed commercial concentrates(control), yeast culture supplemented(0.42%, DM basis) total mixed diet(YC) and brewer`s yeast slurry based(6.36%, DM basis) total mixed diet(BYS). Yeast culture was prepared by muting wheat bran with water in a same weight basis, followed by adding Saccharomyces cerevisiae(Gist-brocade, Delft-Holland) to the mixture, and then by fermenting the yeast containing mixture at 18∼20℃ for 3-5 days. Brewer`s yeast slurry based diet(BYS) was fermented under the same environmental condition as for YC prior to feeding. The results observed from the study are summerized as follows Tendencies of slightly increased tuminal pH but decreased ammonia concentration were observed from YC compared to those from control and BYS. Yeast containing diet(YC and BYS) did not affect total fatty acid production and its molar percent except for decreased butyrate at 30 min.(P$lt;.001) prior to feeding, and valerate at 30 min. (P$lt;.05) and 2 hour(P$lt;.034) postfeeding by BYS feeding. Reduced ruminal degradations of dry matter, crude protein, neutral detergent fiber and organic matter in BYS were observed than those in control and YC. Similar tendency was found in whole tract digestibility of diets. There were no differences in ruminal characteristics of fermentation and degradation, and whole tract digestibility between control and YC. Total viable bacterial number and nitrogen retention were not affected by dies.