Sweet basil (Ocimum basilicum L.) is known locally as "feslegen or reyhan", belonging to the Lamiaceae family and is widely grown as an ornamental, medicinal and spice crops in the Mediterranean countries, including Turkey. Basil is cultivated in small areas like gardens. However this plant must be produced in the shape of field farming for a new crop. Cultivar, cropping season, plant ontogeny and plant part had significant effects on the yield and quality of the essential oil of O. basilicum as with other essential oil plants. The present work was undertaken with the main objective to investigate the essential oil rate and constitutents of essential oil isolated from the aerial parts of O. basilicum cultivated in Diyarbakir as affected by different development stage. The essential oils obtained by hydrodistillation were analyzed by gas chromatography coupled with mass spectrometry (GC - MS) The yield of essential oil from different development stages varied between 1.2-1.6%, and the major components identified in essential oils are linalool, eugenol and terpinen-4-ol. The highest linalool content was obtained from flowering stage and it was observed that linalool percentages decreased in the post-flowering period, which showed an increase towards the flowering period, eugenol was rich in pre-flowering stage and the maximum amounts of terpinolen-4-ol were observed in post-flowering stage.