Abstract: This study reports on the chemical compositions of the essential oil of Rosmarinus officinalis L. (Rosemary) grown in Mersin, Turkey. The essential oil of rosemary was obtained by hydrodistillation method, and the yield of rosemary oil was found to be about 1.2 % (v:w). The hydrodistilled volatile oil was analyzed by gas chromatography and mass spectrometry techniques. Forty-five components were identified in the essential oil of R. officinalis, which represented 100% of the total essential oils. The oxygenated monoterpenes content possessed the highest value, 64.78% of the oil, among which eucalyptol (33.15%) and camphor (10.31%) were the most abundant components. In addition, the oil contained mainly monoterpene hydrocarbons, sesquiterpene hydrocarbons, oxygenated sesquiterpenes, and diterpenes. The least amount of diterpenes were found in the content of the oil. Isopimara-9 (11),15-diene (0.14%) and α-springene (0.06%) were two compounds determined as diterpene compounds.