Rhinacanthins-rich Rhinacanthusnasutus (RRn) extract was prepared and standardized to contain total rhinacantins not less than70%w/w.In this study antimicrobial activities of the RRn extract as well as rhinacanthin-C against Streptococcus mutans Propionibacterium acnes, Helicobacter pylori Staphylococcus aureus, S epidermidis and Candida albicans were evaluated by microdilution assay. It was found that the RRn extract exhibited potent bactericidal activity against S.mutans with MIC and MBC of 4 mg/ml,and potent bacteriostatic activity against S. epidermidis, P.acnes and S. aureus with the MICs of 8–16 mg/ml. However,