Abstract: The objective of this trial was to preliminarily explore and compare the effects of two sources of commercial active yeast additive (Angel Yeast & Lesaffre Yeast) on the characteristics of in vitro fermentation, to predict their potential influences on the production performance of dairy cows, and to provide the regulation of nutrition physiology with theoretical basis. The study was conducted as one-factor block experimental design, and four supplemental levels (0, 0.025%, 0.050%, and 0.075%) were set for each source of active yeast. By employing the method of in vitro fermentation (48 hours), maize straw and rice straw were respectively used as the substrate. The result of this study suggests that the supplementation of active yeast did not exert a significant influence on the GP, indexes of LE model, and NDFD (P>0.05), but it elevated the DMD of maize straw (P<0.05) and rice straw (P<0.01). The supplementation of active yeast exerted a non-significant influence on the pH and NH₃-N of in vitro fermentation of maize straw (P>0.05), but it significantly affected the pH and NH₃-N of in vitro fermentation of rice straw (P<0.05). In the in vitro fermentation of both maize straw and rice straw, the treatment of Lesaffre Yeast significantly raised the DMD compared to the treatment of Angel Yeast (P<0.05). It can be inferred that the effects of Lesaffre Yeast were more desirable than those of Angel Yeast, and the optimum supplemental level of the former was 0.075%.