Statement of the Problem: Nanosilver particles have the potential to serve as a bactericidal agent because of the inherent antimicrobial influences of silver ion. The literature confirmed that specific micro-organisms, especially streptococci, have an important role as an etiological factor for caries.Purpose: The aim of this study was to evaluate the antimicrobial effect of conventional and nanosilver-containing varnishes on oral streptococci.Materials and Method: Pure cultivations of Streptococcus mutans and Streptococcus salivarius were prepared on blood agar media. Thereafter, 0.5 McFarland standard of recently grown bacteria in normal saline was prepared and the bacteria were cultivated monotonously on the culture medium surface by applying a swab. Different concentrations of nanosilver varnishes were prepared in the Mueller- Hinton broth medium in the test tubes and equal amounts of 0.5 McFarland suspension of all the tested bacteria were added separately to all test tubes. A tube without varnish was included as the control sample. The tubes were kept at 37°C for 24 hours, then cultured to determine the numbers of bacteria in each tube by counting colonies. The numbers of bacteria in tubes with varnish were compared to the numbers of bacteria in the tube without varnish. In the instance of observing any reduction in the growth, the minimum inhibitory concentration for growth in the tube with varnish was determined.Results: Nanosilver varnish had an antimicrobial effect on S. mutans and S. salivarius.  S. salivarius was more susceptible than S. mutans to the nanosilver varnish.Conclusion: Based on the results of this study, nanosilver varnishes can be used under amalgam restorations to reduce microbial population and subsequently preventing the recurrent caries.Key Words: Antibacterial effect; Nanosilver; Oral streptococci