Pathogenic microbes are the most common cause of chronic infections and mortality in mammals and loss of agricultural crops in the world. Antimicrobial agents including antibiotics and antifungals are often used in the treatment of infections due to their exceptional consequences faces various problems that limits the treatment by them. Therefore, herbal sources are attracting more attention due to lower side effects and in some cases better and faster effects. Plant essential oils (EOs) have many antimicrobial and pharmacological effects, but EOs are volatile, heat-sensitive and water-insoluble compounds that limit their use, encapsulating EOs can improve the properties of such compounds. Chitosan, a biodegradable nanopolymer, is very important in drug transfer due to better encapsulation, controlled release and low toxicity. Therefore, this research was conducted to investigate the encapsulation technique as a suitable method to preserve essential oils and increase their antimicrobial properties. First, the chitosan polymer was synthesized and used to encapsulate rosemary essential oil after examining its shape and size by spectroscopy (FTIR) and scanning electron microscopy (SEM). Then, the antibacterial effect of encapsulated EO and free EO was investigated by liquid microdilution method. The results showed that encapsulated EO had a greater antibacterial effect against gram-negative bacteria and fungi compared to free EO.