Abstract: The pathogenic bacteria from the dead bodies of the lesions in organs and tissues of tibetan mastiff was isolated and cultured, according to the diagnosis of bacterial isolation and culture, staining, microscopy, biochemical test, drug sensitive test and the results of 16S rRNA gene sequence analysis, the pathogenic bacteria was proved to be Streptococcus canis. The results of antibiotic susceptibility testing demonstrated that, the Streptococcus canis was highly sensitive to ceftiofur sodium, roxithromycin, tilmicosin, ofloxacin, lincomycin, ampicillin, medium sensitive to doxycycline, resistance to amikacin. Their 16S rRNA was closely related to Strephtococcus canis（CG40） with homologies of 99.2%.

Key words: tibetan mastiff, Streptococcus canis, isolation, identification