Mastitis is an inflammation of the parenchyma of mammary gland regardless of the causes. Mastitis is characterized by a range of physical and chemical changes in the glandular tissue. The most important changes in milk include discoloration, the present clots and the present of large number of leucocytes. There is a swelling, heat, pain and edema in mammary gland in many clinical causes. Coagulase positive S. aurousis a major pathogen of the bovine mammary gland and a common cause of contagious mastitis in cattle. The aim of this study was the evaluation of outbreaks of Staphylococcus aurous mastitisand its sensitivity to Co-amoxiclavein Tabriz dairy herd. This study is conducted in ten dairy herd of Tabriz on about 1,000 cows. After doing CMT and identifying of infected cows, the milk samples obtained from infected quarters and transport to microbiological laboratories. After microbial culture of milk samples and isolated of S. aurous, antimicrobial sensitivity test with disk diffusion method perform by co-amoxiclav, penicillin, oxytetracyclin, co-trimoxazole, lincomycin and cefquinome. This study defined that the outbreak of subclinical coagolase positive staphylococcus mastitis in Tabriz dairy herd was 4.43% (3.35% S. aureus and 1.08% S. intermedicus). The antimicrobial sensitivity test shown that 41.17% of Staphylococcus aureus isolated from bovine mastitis in Tabriz dairy herd were susceptible to co-amoxiclav, 94.1% to cefquinome, 85.29 to sulfonamides (Co-trimoxazole), 32.35 lincomycin, 14.7% to oxytetracyclin and 5.88% to penicillin. Results of this study defined that Co-amoxiclav has a moderate effect on Staphylococcus aureus isolated from subclinical bovine mastitis in Tabriz dairy herd. It seems that cefquinome and sulfonamides has a great effect on Staphylococcus aureus isolated from subclinical bovine mastitis at in vivo.