Abstract: Streptococcus agalactiae is one of pathogenic bacterium that inducing mastitis of dairy cow. In order to master the popularity and laid the reference for selection of medicine to treat mastitis, 258 samples were collected from 16 dairy cow farms in Qinghai province, and 137 Streptococcus strains were isolated,including Streptococcus agalactiae 42 strains after sub-culture, gram straining, plasma-coagulase test, CAMP test, biochemical tests and animal pathogenicity tests. The results indicated that Streptococcus agalactiae was the primary pathogenic bacteria of mastitis. All of Streptococcus agalactiaes were hypersensitive to cephradine, and partially resistant to amikacin ofloxacin, ampicillin and sulfanilamide group after K-B medicine sensitivity test.

Key words: dairy cow; mastitis; Streptococcus agalactiae; isolation and identification