Oregano (Origanum vulgare L.), a native plant from Iran, belongs to Lamiaceae family. This species is traditionally utilized as sedative, diuretic, sweater and antiseptic, It is also used to treat of gastrointestinal diseases and constipation. Origanum species are widely used in spice industry. In this study, the essential oil of the aerial parts of a subspecies from this plant namely Origanum vulgare ssp. vulgare was isolated through hydro-distillation following its collection from South region of Chalus in its flowering as well as in seed stages. The extracted essential oils were analyzed through GC and GC/MS systems. Nienteen compounds were identified in the extracted essential oil in the flowering stage which constituted about 99% of the total oil. The latter sample was found to be rich in linalyl acetate (27.2%), g-terpinene (16.5%), 3-octanone (10.9%), b-pinene (8.4%) and carvacrol (6.4%). The 22 identified compounds made up 98.6% of the essential oil in seed stage, carvacrol (23.2%), a-pinene (15.8%), b-pinene (10.7%) and trans-caryophyllene (5.3%) constituted the main compounds. According to the GC/MS results, the monoterpenes were characterized as the most predominant essential oil fraction in either stage. The percentages of the other components of the oils were 16.8 and 5.7 for the plant at flowering and at its seed stages, respectively.