Abstract: The aim of the present study was to determine the dominant species of bacteria associated with bovine mastitis in Tongliao City of Inner Mongolia. Three quarter milk samples were collected from dairy cows with clinical mastitis and were used to isolate pathogenic bacteria by conventional method. The antimicrobial susceptibility test of the isolates was conducted using Kirby-Bauer disk diffusion method. The results revealed that a total of 14 strains were isolated from the 3 samples and they were subsequently phenotypically identified as Streptococcus agalactiae. While reduced susceptibility to streptomycin and sulfadiazine were observed, kanamycin, gentamicin and midecamycin are found as susceptible drugs for the isolated Streptococcus strains.