Abstract: A Streptococcus agalactiae GS was isolated from milk from mastitis cattle in Gansu region of China. Sequence and phylogenetic analysis of Sip gene segments revealed that the Streptococcus agalactiae GS was most similar to a recent group B streptococcus isolated in dairy cattle from China. The Sip gene were selected and amplified, it contained a 1305 bp Open Reading Frame (ORF) which encoded 434 amino acids. The molecular mass of the deduced amino acid sequences was 56 kD. Blast analysis showed that it shared high identities (91.0-99.0%) with Sip sequences of other GBS registered in GenBank while lower identities (<50%) was found compared with other species of Streptococcus. The deduced amino acid homology was 90.2-98.8%.