Objectives The present study was carried out to evaluate the effect of Actinomyces bacteria on dental caries formation on rats. Materials and methods The present study was carried out on forty male Albino rats at range of 21 days of age. The rats were selected and divided into two groups, including 20 rats of each, group I was received Actinomyces isolates beside cariogenic diet. However, group II was received only cariogenic diet to the end of the experiment. Both groups were orally infected with Streptococcus mutans for 3 consecutive days. After 3 months all rats were sacrificed, the mandible was examined using magnifying lens to record caries incidence. χ 2 test was used for statistical comparison of caries incidence between the two groups. Level of significance was 0.05. Results Dental caries formation was significantly decreased in the study group compared to the negative control group. Conclusion Actinomyces administration may be effective in reducing dental caries formation on rats, suggesting the role of dental plaque micro-organisms in formation of pigment and reduction of dental caries.