Abstract: This study aimed to isolate and identify facultatively anaerobic cellulolytic bacterium in the rumen of yaks and to provide a basis for studying new types of silage additives. Fresh ruminal fluid was collected from Tibetan yaks and highly cellulolytic facultatively anaerobic bacteria were screened out by using Congo red staining and liquid secondary screening culture media. Cellulolytic bacteria strains were identified based on their morphological, physiological and biochemical characteristics, as well as by 16S rRNA analysis. Time course profile of growth and enzyme production by strain JF85 was obtained. The degradation efficiency of rice straw was examined by evaluating cellulose, hemicellulose and lignin contents under a range of condition. Strain JF85 was identified as Enterococcus faecalis. This strain grew well across a wide range of conditions: temperatures of 15-55 ℃, pH levels of 3.0-7.0 and NaCl levels of 3.0% and 6.5%. It exhibited the maximal endoglucanase activity (0.41 U/mL) and filter paper activity (0.13 U/mL) after 36 hours of incubation. In simulated fermentation, compared to the control, adding the JF85 strain significantly decreased the dry matter, cellulose and hemicellulose contents of rice straw after 14 days of inoculation (P<0.05). JF85-treated rice straw showed its highest water soluble carbohydrate content after 7 days of inoculation. However, there were no significant differences in lignin contents among all treatments. Applications of the JF85 strain, with its highly cellulolytic abilities and tolerance of acid and salt, have the potential to significantly improve the production of silage.