Chitosan nanoparticles were studied as antimicrobial agent. The antibacterial activity of chitosan nanoparticles were investigated against three Gram-negative bacteria; Escherichia coli, Pseudomonas aeruginosa and Salmonella typhi, and three Gram-positive bacteria; Staphylococcus aureus, Enterococcus faecalis and Streptococcus pyogenes. The antifungal activity were examined against three fungi; Geotrichum candidum, Candida krusei and Candida parapsilosis. The antiviral activities were tested against three viruses; Rift Valley Fever (RVFV), Herpes simplex- 1 (HSV-1) and Coxsackie viruses. Chitosan nanoparticles were inhibited all bacteria and fungi except E. faecalis seemed to be resistant strain. Infectivity titers of all viruses were reduced by chitosan nanoparticles, which are a natural antimicrobial agent.