This experiment was conducted to study the changes of forage nutrient content and seasonal shift of bacterial community in rumen of the grazing Tibetan sheep. PCR and denaturing gradient gel electrophoresis (DGGE) were used to monitor changes of bacterial community in rumen of 12 Tibetan sheep. The V6～V8 regions of the 16S rDNA of rumen bacteria were amplified. DGGE profiles of the PCR amplicons were compared by similarity analysis. Shannon diversity index of the bacterial community was also performed. Results showed: 1) in autumn, the structural diversity of bacterial community was significantly higher than that in winter (P<0.01), spring (P<0.01) and summer (P<0.05) and there were no significant differences among winter, spring and summer (P>0.05); 2) rumen bacterial community changed with seasonal shift. Similar rate between autumn and summer was higher than 90%; similar rate between autumn and winter was less than 70% and similar rate between spring, summer and autumn, winter was less than 60%; 3) the CP and DE content of forages in summer were significantly (P<0.01) higher than those in other seasons, but the pH of rumen in summer was significantly (P<0.01) lower than that in other seasons, and the structural diversity of the bacterial community also reduced. The CP and DE content of forages in autumn were significantly (P<0.05) higher than that in winter and spring. The pH of rumen in autumn were from 7.1 to 7.3, and the structural diversity of the bacterial community were higher than that in other seasons. It was concluded that bacterial community in rumen of the grazing Tibetan sheep changed with seasonal shift, which were affected by pH in rumen, rumen physical characteristics and nutritional ingredient.［Chinese Journal of Animal Nutrition, ２００9，２1（5）:798-802］