Abstract: (Objective) The object of this study was to estimate the effect of the aflatoxin B1 on the metabolism of rumen microorganisms by some fermented parameters such as pH, volatile fatty acid, NH3-N, the composition of gas, and the activity of several cellulose. (Method)The experiment was to determination the active change of rumen flora in the short-term fermentation system which had increased different level of aflatoxin B1(0μg/mL，0.1μg/mL，1μg/mL，10μg/mL). (Result)The result of this study found that the concentration of NH3-N and volatile fatty acid were decreasing following the increasing of aflatoxin B1. The production of volatile fatty acid based on the substrate of maize meal was higher (p<0.05)than of alfalfa hay. The proportion of acetate, propionate, butyrate and valerate also decreasing according to the increasing the dose of aflatoxin B1, and the proportion of iso-valerate increasing because of high level of dimethylsulfoxide(DMSO). Aflatoxin B1 also caused inhibition the methane production (p<0.05) . Beside it has ability to inhibit the activity of cellulose. (Conclusion)According to the above results, It shows clearly that aflatoxin could be inhibition the activity of microorganisms in rumen, especially the fermentation of fibrous feedstuff.