The study investigated the antibacterial activity Effect Of Nano-Chitosan Against Streptococcus Mutans, It was collected from patients with periodontal disease and dental caries, and identification of bacteria and Antibiotic susceptibility tests were performed by using the VITEK2 compact system. This retrospective study was conducted at the Ba'aqubah teaching hospital.in duration February/ 2022 - April 2022 -, Data have been collected from coming patients at Specialized Dental Centers in Diyala, A total of 50 specimens, 35 ( 70%) were males, and 15 (30%)were females, the growth samples appeared in 17 ( 34%) isolates from dental caries and strep. mutans appeared in 10( 20 %) isolates. The highest rate of antibiotic resistance was observed in 3(6%) of Strep. mutans. Then the effect of Nano-chitosan on antibiotic-resistant isolates was studied by determining the value of M I C.