Mastitis is rare in mares compared with cattle, a mare suffered from mastitis was referred to the Veterinary Teaching Hospital, College of Veterinary Medicine, University of Mosul for examination with a history of abortion before two months and gradually loss of body weight. Clinical examination revealed, a swollen left mammary gland, painful to palpate, there was mild degree of edema in the tissue surrounding the gland, fluid with clot expressed from the gland, body temperature was 37.4ºC and heart rate was 40/ minute. Fluid (mammary secretions), milk samples were collected under sterile condition and submitted to the laboratory for bacterial culture and identification test. The results of the culture and biochemical tests showed that the Staphylococcus spp was the etiological agent. This case was treated with systemic and local antibiotic therapy (Penicillin preparation).