Objectives: The present invitro study was carried to evaluate the antibacterial effect of silver diamine fluoride 38% (SDF) and sodium fluoride varnish 5% (NaF) on inhibition of bacterial biofilm formation subjects and methods: a total number of 48 biofilms were classified into three main groups (16 for each) and treated with. Group A: SDF 38% for 1 minute only per day for three consecutive days and biofilms were washed 3 times with fresh media. Group B: NaF Varnish 5% for 1 minute only per day for three consecutive days and biofilms were washed 3 times with fresh media. Group C: left untreated (control group). After treatment, fresh media was added to biofilms that were grown at 1st day and at 3rd day . Streptococcus mutans (S. mutans ) and Lactobacillus acidophilus (L. acidophilus) were grown anaerobically. Colonies were counted and colony forming unit (CFU) per mL of was calculated. Results: There was a statistically significant difference in the count of S. mutans and L. acidophilus between all groups in which the highest bacterial count was found in Group C group followed by Group B while the least bacterial count was found in Group A. Thus results recorded SDF (82%,83%) higher reduction than NaF (70%,74%) according to S.mutans at 1st and 3rd day respectively and SDF (89%,88%) higher reduction than NaF (74%,81%) according to L acidophilus at 1st and 3rd day respectively. Conclusion: SDF is a non-invasive, simple and low-cost approach to arresting dental caries.