Inula viscosa was chosen to start a research due to the fact that this plant spreads widely in different geographical regions in Syria. The effect of environmental conditions on the amount of essential oil extracted from aerial parts of Inula was determined during three phonological phases (vegetative- flowering- post flowering). The plant was collected in the year 2013from three different geographical regions in Syria (Beit Diebeh, Der Mamma and Kafra). The results showed that the full flowering stage is ideal to harvest the plant in order to achieve greater therapeutic benefits. On the other hand, the amount of essential oil was very little during the vegetative stage in the three different regions. Climatic factors as moisture were very essential to biosynthesize larger amounts of essential oil, where the highest ratio reached 2.3 ml/ 100 g in (Beit Dieneh- Tartous).