In recent years, due to some problems caused by using chemical pesticides, there is a strong tendency to use the potential of biological substances in controling pests, diseases and weeds. Citrus essential oil is a mixture of volatile compounds, which are widely used in food and pharmaceutical industries due to their antifungal activity. Melanin is a high molecular weight pigment that is produced by animals, plants and microorganisms. Melanin produced in fungi, especially Alternaria, protects them against environmental stresses, by producing a envelope around fungal structures, indirectly plays a role in its pathogenicity. In addition, by production of a structural layer around the appressorium and contribution to its function, as well as a scavenger of reactive oxygen species, which is a part of the host defense system, is combined with them at the pathogen´s entry point and in this way helps to the pathogenicity of the fungus. In this research, efficiency of four orange cultivars essential oils including Thomson Navel, Yapha Shirin, Sangin and Valencia in controlling melanin production in three different species of Alternaria pathogenic on citrus, including A. alternata, A. atra and A. dumosa were studied. The experiments were conducted in vitro via mixing the essential oils with PDA medium at concentrations ranging from 0 to 4000 ppm with three replications. After 7 days at 27 ± 1 ° C, the results showed that essential oils of all four cultivars significantly reduced the intensity of melanin production Therefore, it seems that the oils also reduce the fungal pathogenicity. Due to the safety of the application of essential oils and their compounds for humans and the environment, it is likely that the essential oils or their constituents can be used to control plant pathogenic fungi, or for production of new fungicides.