Abstract: Three yeast culture (YC) products, YC-1, YC-2 and YC-3 were fed respectively to the beef cattle with permanent rumen fistula.We studied the effects of these YC products on rumen fermentation, activities of fiber hydrolytic enzymes and quantities of predominant fibrolytic bacteria. With YC-2 supplement, the concentrations of total VFA, acetate, propionate and butyrate were all increased significantly (P＜0.05). The acetate to propionate ratios were significantly decreased (P＜0.01) with YC-1 and YC-3 supplement. The activities of CMCase, Salicinase and Xylanase in the rumen were all increased significantly (P＜0.01) with three kinds of YC supplements, and the relative population of R. flaveciens was also increased significantly. By hybridization analysis, sum of the relative populations of cellulolytic species was about 3.80%±0.2%.