Abstract: Quorum sensing is a process of cell- to- cell communication in Streptococcus mutans which secretes a special signal molecule, then induces and actives the expression of related genes, in order to coordinate the biological behaviors of Streptococcus mutans. Recently a new signal molecule, AI- 2, is regarded as a general signal molecule, involoving in communication among oral bacteria, regulating the stability of biofilms. In this review, we summarize the previous researches on the signal molecule AI- 2 in Streptococcus mutans in recent years.