The richness and diversity of the Moroccan Saharan flora constitutes a veritable phytogenetic reservoir with a flora that includes more than 500 aromatic and medicinal species, many of which remain unexploited. Among these plants we find Atremisia herba alba in the region of Guelmim (Sahara of Morocco) which has a great potential in phytomass and which deserves to be better valued. The objective of this work is the comparative study of the yield and chemical composition of the essential oil derived from Artemisia herba alba in Guelmim region (Sahara of Morocco) according to the date of harvest as well as the determination of its antibacterial and antifungal activity towards eleven microorganisms. Essential oils yields expressed in ml compared with 100 g of dry matter of Artemisia herba alba harvested in March, June, September and December (2015) are respectively 1.62; 0.95; 0.64 and 1.85 %. Chromatographic analyses of essential oils revealed the predominance of camphor, cis-b-terpineol, camphene, trans-thujone and trans-verbenol whose rates vary according to the plant phenological stage. The essential oil of Artemisia herba-alba collected during March was the most active against all microorganisms tested.