An in vivo fermentation experiment (Expt.1) and a digestibility and growth trial(Expt.2) were conducted to determine the effect of levels of Yucca Schidigera Extract (YSE) on ruminal fermentation parameters ,digestibility of nutrients and growth performance in Sheep. Three same levels of YSE supplementation were studied in both experiments. In the Expt.1,four rumen fistulated male sheep with initial body weight of 33±1.8 ㎏were randomly assigned according to a 4×4 Latin square design. The dietary treatments were YSE based offering at 0,100, 200 and 300 mg/kg diet. It was found that ruminal pH was not significantly different among treatments. Relative to control, ruminal propionate concentration was increased by YSE addition in a dose-dependent manner by up to 29.79%（P<0.05）,the acetic concentration was decreased by up to2 1.23%（P<0.05）.Ruminal ammonia concentration was larger (P<0.05) in sheep receiving no YSE (increased by 18.86 mg/dl) than in those receiving 200mg/kg (2.85 mg/dl increase in NH3) or 300mg/kg (2.72 mg/dl increase). Protozoa populations in the rumen were lower (P<0.05) with YSE of 200-300 mg/kg than without. In the digestibility and growth trial(Expt.2),increasing levels of YSE resulted in liner increase in daily body weight gain rate (P<0.05) and feed conversion efficiency(P<0.05). Additionally ,apparent digestibilities(%) of DM,OM,CP,and NDF were significantly different in all treatment .In conclusion, the 200 and 300 mg/kg YSE groups have a particular suppressing effect on ruminal ammonia concentration, ammonia N concentrations and protozoa populations were decreased, The effects on ruminal propionate and acetic concentration were probably a result of a selective inhibitory effect of YSE on rumen microbial species. The effect of YSE on ruminal ammonia concentration likely resulted from a decreased population of protozoa, presumably, from ammonia binding by YSE. YSE can significantly improved the apparent digestibility of nutrient and sheep growth performance.