Mastitis, a symptom of inflammation in mammary tissue by infection with various kinds of bacteria, causes huge economic losses in the milk industry. One of the popular methods for treatment of mastitis is antibiotics, although this prohibits milk shipping and sometimes causes resistant microbes. Therefore, a new strategy to treat mastitis without antibiotics is eagerly required around the world. Antimicrobial factors belong to innate immunity and can start their function extremely early after bacterial stimulation. These factors have antimicrobial activity for a broad spectrum of bacteria. Elucidation of causal mechanisms and functions of antimicrobial factors in the mammary gland is thought to result in suitable methods for prevention and treatment of mastitis. Therefore, this review introduces traits of some antimicrobial factors and the mechanisms for expressing, producing and secreting them in the mammary gland. For antimicrobial factors, lingual antimicrobial peptide (LAP), S100A7, cathelicidin and lactoferrin are controlled in different sites and different time courses, suggesting that antimicrobial factors play different roles for local defense against bacterial infection in the mammary gland. These findings will contribute to the development of prevention and treatment methods for mastitis.