Essential oils are composed of various chemical compositions in different parts of plants that play an important role in their biological activities such as antimicrobial activity. So, they can use in different industries as well as control molds, various fungi and bacteria such as food-borne and phytopatogenic organisms which are caused different postharvest diseases, animal and human disorders. Moreover they can use as preservative in food industry. These activities of essential oils are associated with phytochemicals components. In addition to this, percentages inhibition of mycelia growth against microorganisms depend on some other factors such as the screening antimicrobial methods, the day of observation, oil concentration and examined fungal and bacterial species. The aim of this chapter is studying the biological activity of essential oils such as antimicrobial to reduce the hazardous effects of synthetic fungicidal and bactericidal products, the increasing interest in the possible application of essential oils for pathogens control and preservative application have aimed to investigate new sources of biologically active natural products in different industries.