Abstract: In order to study the effects of exogenous cellulase on the bacterial degradation by rumen fiber of dairy cow,three dairy cows with close ages and weight were selected as experimental animals to install permanent rumen fistula.The self-control test method that was consisted of two experimental periods was conducted.During the first period,no exogenous cellulase was supplemented to the basal diet （control group）.During the second period,exogenous cellulase combination was supplemented to the basal diet at the proportion of 0.1% （experimental group）.The results showed that exogenous cellulase significantly increased the number of Ruminococcus albus,Ruminococcus flavefaciens,Fibrobacter succinogenes,Butyrivibrio fibrisolvens,Eubacterium ruminantium and Prevotella ruminicola in the rumen of dairy cow（P0.05～P0.01）.