Hyssop (Hyssopus officinalis L.) is an aromatic and medicinal perennial herb native to Southern Europe and temperate regions of Asia. The study aimed to determine the effect of different plant parts harvested at different flowering stages: pre-flowering, full-flowering and post-flowering, in some agronomical characteristics, essential oil yield and components of hyssop plant in semi-arid climatic conditions of South Eastern Anatolia, Diyarbakir, Turkey. The results showed that both flowering stages and plant parts had important effects on fresh, dry herbage, dry leaf yield of hyssop, as well as essential oil content. It was concluded that under semi-arid climatic conditions, hyssop plant could be grown successfully in conformity to the limits of ISO standards and harvest from top parts of the plants at full flowering stage could be preferred for maximum dry leaf yield and essential oil contents.