Chlamydia plasmid is a 7.5 kb, highly conserved, non-integrative DNA molecule present in almost all strains of Chlamydia trachomatis, as well as Chlamydia muridarum and Chlamydia psittaci. Recently, Chlamydia plasmid has been considered to be a virulence factor which contributes to mouse oviduct hydrosalpinx. Studies in animal models indicate that plasmid-deficient Chlamydia strains function as live attenuated vaccines against genital and ocular Chlamydia infections. Nevertheless, Chlamydia plasmid is also a potential genetic manipulation tool for revealing Chlamydia trachomatis pathogenic mechanism. Therefore, research on Chlamydia plasmid is very meaningful.