Abstract: On the same dietary ratio of forage to concentrate（R/C）of 8:2, experimental animals were divided into H group(GI 2.89)、M group(GI 1.73) and L group(GI 0.65) according to Grading Index(GI) of forage, to study the effect of different quality of forage on rumen fermentation and quantity of predominant fibrolytic bacterial population. The results showed that the pH values of the animals in three groups were all in normal range, the interclass differences were not significant（p>0.05）,and the change of H group’s pH values was more mild. The ruminal NH3-N concentration display ascendant tendency,in the animals of H group was more significant than L group（p<0.05）,and the ruminal concentration of acetic acid had reducing tendency, the concentration of propanoic acid had ascendant tendency, the acetate to propionate ratio had reducing tendency, the mean value’s differences of the ruminal concentration of total VFA concentration among three groups were not significant（p>0.05）. By hybridization analysis, the relative amount of R.flaveciens﹑R.albus and B.succinogenes in the rumen of the sheep were 1.18％、2.44％and 10.07％, respectively.

Key words: the quality of forage, rumen fermentation, oligonucleotide probe, cellulolytic bacteria, ruminal bacteria