The koumiss is a kind of fermented milk drink by multi microorganisms. Microorganisms are isolated from 15 koumiss samples of pastoral area in Xilin Guole. According to the morphological characteristics and biochemical reacton and sugar fermentation characteristics, they respectively belonged to Lactococcus 5 genus, Lactobacillus 6 genus, yeasts 6 genus. It is also found that some microorganisms have inhibitory effect on the growth of Listeria, S. aureus and E. coli . Nine strains of Lactococcus and 12 strains of Lactobacillus have the inhibitory effect on Listeria, but do not have the inhibitory effect on E. coli and S. aureus. Four strains of yeasts have the inhibitory effect on E. coli, two strains of them have the inhibitory effect of S. aureus, meanwhile , but do not have the inhibitory effect on the Listeria.