The supercritical fluid is usually Carbon dioxide because is not toxic, environmentally safe, no falmmable, operation under mild conditions (p>74 bar and T>310C), physiologically harmless, with low cost, non-explosive, and readily available and it can be easily removed from products. Nowadays, supercritical fluid extraction , is used as an attractive extractin method to conventional liquid extraction in wide variety areas including the industries of food, pharmacy, environmental engineering, chemical and oil industries. The essential oil of Achillea wilhelmsii obtained by using supercritical carbon dioxide (sc-co2) extraction were investigated and the obtaind crude extract yields were compared in order to select the best operation parameters. The studied parameter were pressure (150, 170 and 190 bar), temperature (35, 40 and 450C) and dynamic extraction time (10, 30 and 40 min).