The essential oil of the aerial parts of Rosmarinus officinalis collected from Konya, Turkey was analysed by gas chromatography and gas chromatography–mass spectrometry. The oil yield of dried plant (volume/dry weight) obtained by hydrodistillation was 1.9%. Twenty compounds representing 99.93% of the oils were identified. The main constituents of the oils were p-cymene (44.02%), linalool (20.5%), γ-terpinene (16.62%), thymol (1.81%), β-pinene (3.61%), α-pinene (2.83%) and eucalyptol (2.64%). The oil consisted of monoterpenic hydrocarbons, oxygenated monoterpenes and sesquiterpene hydrocarbons. Also, the inhibition effect of rosemary oil was investigated against Alternaria alternata, Botrytis cinerea and Fusarium oxysporum. The experiment was carried out in vitro using disc diffusion to investigate the antifungal action of the oil. Oil tested on potato dextrose agar plates exhibited an inhibitory effect. The extent of inhibition of fungal growth varied depending on the levels of essential oil used in experiment.