The antimicrobial effects of silver (Ag) ion or salts are well known. Recently, silver nanoparticle is attracting an interest in a wide variety of fields since it has been known to be safe and effective as an antimicrobial agent against a broad spectrum of microorganisms. Although silver nanoparticle has been applied to various kinds of products owing to its potent antimicrobial activity, the effects of silver nanoparticle on microorganisms and antimicrobial mechanism have not been revealed clearly. In this paper, we summarized the characteristics, antimicrobial activities and mechanisms, cytotoxicity and applicability of silver nanoparticle.