Mastic, an important evergreen and medicinal plant grown in the forest of Al-Jabal Al-Akhdar, Libya has suffered heavy losses due to a leaf disease. Various kinds of leaf spots and blight were found on plant leaves. The objective of this work was to study fungi based on morphological characterize and to identify those that are pathogenic. Disease incidence were quite variable between seasons. Rate of diseased leaves was recorded about 58% in spring while 24.3% in winter season. Four fungal species were frequently isolated from the disease symptoms. Alternaria alternata was the most predominant fungus associated with the brown leaf spot on mastic, followed by Pestalotiopsis fici was commonly isolated from leaf blight and P. guepinii and P. palmarum were isolated from leaf tip death and silvery gray leaf spots respectively. Koch's postulate was verified by inoculating healthy mastic leaves. The literature indicates that this is the first report of a leaf disease of mastic in Libya.