objective:to investigate the antibacterial activity of grapefruit seed extract mouthwash against streptococcus mutans and explore the potential mechanism. methods:the minimal inhibitory concentration (mic) and the minimal bactericidal concentration(mbc) of the grapefruit seed extract mouthwash against s.mutans were detected using double dilution. and the cell wall alteration of s.mutans treated with grapefruit seed extract mouthwash was observed by using a scanning electron microscope(sem). results:the mic and mbc determined for grapefruit seed extract mouthwash were 46.875 mg/l and 93.75 mg/l. both rupture of the cell wall and release of the intracellular components were observed in the grapefruit seed extract mouthwash-treated group by sem. conclusion:the grapefruit seed extract mouthwash can inhibit the growth of s.mutans and kill them in vitro,this suggests that grapefruit seed extract mouthwash has an antibacterial action on the s.mutans by affecting the normal structure and function of the cell wall of s.mutans.