Abstract: Streptococcus agalactiae is one of common pathogens causing mastitis in dairy cows. It can affect goats and sheep causing chronic mastitis, and cause infant septicemia, meningitis and pneumonia. It belongs to group B streptococcus bacteria in clinical medicine. In the present study, 120 milk samples were collected form the cows suffering from mastitis in five dairy farms around Hohhot. Through sub-cuhure, morphological observation and biochemical tests, 14 strains of Streptococcus agalactiae were identified, and meanwhile the drug sensitivity and animal pathogenicity tests were carried out. The results of the drug sensitivity test showed that all the Streptococcus ogalactiae strains were highly sensitive to penicillins and aminoglycosides grugs, and moderately susceptible to sulfa, norfloxacin, quinolones drugs. The results of animal pathogenicity tests showed that the fatality rate of all the strains in mice was more than 80 %.