Abstract: The present research aimed to study the inhibitory effects of garlic (Allium sativum) and onion (Allium cepa L.) extracts on microorganism isolated from dental caries plaques of Jordanian patients. Dental caries plaques were collected in a private clinic then processed and worked upon in a university laboratory. Dental plaques from 91 patients were obtained and grown on plates following standard protocols. A total of 116 isolates were grown. E. coli, Lactobacillus acidophilus, Candida albicans, Streptococcus mitis, Actinomyces naeslandii, Enterococcus faecalis and Streptococcus oralis were decreasingly the most frequent organisms. Standard and different dilutions of garlic and onion extracts were tested against those isolated microorganisms. Median values for zones of inhibitions were calculated and blotted. Garlic extracts consistently showed significantly higher inhibitory effects than onion extracts. This study concludes that Garlic extracts do have significant inhibitory effects against microorganisms associated with dental caries and further studies are needed to explore potential therapeutic uses for such extracts. It also showed that microorganisms associated with dental caries of Jordanian patients are similar to those encountered in other different areas of the world.