Defensin is a cysteine-rich, low molecular weight and cationic antimicrobial peptide. Human breast milk contains high level of defensin. The defensin in human breast milk can not only inhibit the growth of various pathogenic microorganisms, but also promote the development of infant immune system and reduce the incidence of infantile upper respiratory infection. Nowadays, the roles of defensin in breast milk is more recognized. This paper focuses on the molecular characteristics of human defensin, its antibacterial and antiviral roles, and the latest progress in defensin research.