This study aimed to demonstrate the antimicrobial activity of crofton weed oil against cariogenic
bacteria and oral Candida species. The antimicrobial sensitivity testing was determined by a disk diffusion method against Streptococcus mutans, Actinomyces viscosus, Lactobacillus casei and oral Candida species: Candida albicans, Candida glabrata, Candida krusei, Candida tropicalis, Candida parapsilosis and Candida dubliniensis. Additionally, scanning electron microscopy (SEM) was used to investigate microbial ultra-structure alteration after the oil treatment. The oil exhibited similar antibacterial activity as 0.2 % chlorhexidine gluconate (0.2 % CHX) against all tested bacterial strains. The zones of inhibition produced by the oil against S. mutans, A. viscosus and L. casei were 11.1 ± 1.05 mm 18.7 ± 1.81 mm and 10.4 ± 1.55, respectively. However, 0.2 % CHX had greater antifungal activity against C. glabrata, C. krusei, C. parapsilosis and C. dubliniensis than the oil. Neither reagent demonstrated antifungal effect on C. albicans. Unexpectedly, the antifungal effect on C. tropicalis of the oil (19.0 ± 1.00 mm) was nearly two folds higher than that of 0.2 % CHX (11.0 ± 0.0 mm). SEM of microbial cells treated with the oil showed perforations on the cell surface or alteration of cells shape. Crofton weed oil exhibited antimicrobial activities against caries-related bacteria and most oral Candida species, especially C. tropicalis, by affecting microbial cell surface.