This study aims to evaluate the effect of anti-Streptococcus mutans IgY gel on quantity of S. mutans on rats’ tooth surface. Sprague Dawley rats were exposed intra-orally with S. mutans Xc and were fed a caries-inducing diet 2000. The 24 rats were divided into four groups: group A had their teeth coated with IgY gel; group B received sterilized water as a control; group C had their teeth coated with IgY gel starting on the 29th day; and group D had their teeth coated with a gel without IgY. Plaque samples were swabbed from the anterior teeth for S. mutans colony quantification, and saliva was collected to measure immunoreactivity by enzyme-linked immunosorbent assay. The results indicated that the quantity of S. mutans in rats treated with IgY gel showed significant difference compared with the controls. After coating with IgY anti-S. mutans gel, the mean immunoreactivity in rat saliva was higher than that of the no treatment group. In conclusion, topical application with anti-S. mutans IgY gel reduced the quantity of S. mutans on the tooth surface.