The in vitro antioxidant and antimicrobial activities of the essential oil and methanolic extract of Achillea sieheana Staf. (Asteraceae) were investigated in this study. The chemical composition of the essential oil isolated by hydro-distillation from the aerial parts of A. sieheana was analyzed by GC-MS. Camphor (43.36%), Artemisia ketone (25.95%), 1.8-cineole (6.29%) and camphene (4.77%) were the main components in the essential oil. Their antioxidant activities were also evaluated using phosphomolybdenum, beta-carotene bleaching and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assays. A. sieheana methanolic extract showed an effective DPPH scavenging activity (IC50 = 87.04 mu g/mL). The extract had also a high reducing effect (71.08%) on the oxidation of beta-carotene. In addition to evaluating the antioxidant activity of this plant, the total phenolic and flavonoid contents were measured in the extract. The antimicrobial activities of the methanolic extract and the oil were also tested against 13 bacteria and two yeasts. The results showed that both had strong antimicrobial activity against the tested microorganisms.