Our work aims to study a very aromatic and medicinal plant known by the locals called Thymus fontanesii (Thyme). The extraction of essential oil of the plant was carried by a drive to steam. The yield is attractive even in terms of industrial exploitation. A number of physicochemical characteristics were determined on freshly extracted oils. The study of the degradation of these oils as a function of time showed a high instability when stored at room temperature and in the presence of light and rather good preservation of the low temperature and protected from light.