In this study, the essential oil content and composition of Achillea biebersteinii Afan. at different phenological stages including vegetative, floral budding, full flowering (leaves, stems and flowers) and fruit set were studied by means of gas chromatography (GC) and gas chromatographyâmass spectometry (GCâMS). All oil samples from different plant parts and phenological stages were mostly made up of monoterpenoid compounds (88.6 â 99.6%), especially oxygenated ones (52.4 â 82.4%). The oil of the vegetative stage contained high amounts of limonene, 4aα-7β-7aα-nepetalactone, p-cymene and 1,8-cineole. The major constituents in the flower budding stage oil were found to be limonene, 1,8-cineole and 4aα-7β-7aα-nepetalactone. In the oil of the fruit set stage, γ-terpinene, p-cymene and cis-chrysanthenyl acetate were the predominant constituents. On the other hand, the most important compounds from the stem oil were 4aα-7α-7aα-nepetalactone, 1,8-cineole, 4aα-7β-7aα-nepetalactone and camphor. 4aα-7α-7aα-nepetalactone, limonene, 1,8-cineole and cis-p-menth-2-en-1-ol were found in high concentration in the oil of leaves, whereas 4aα-7α-7aα-nepetalactone, 4aα-7β-7aα-nepetalactone, limonene and p-cymene were present in large amounts in the oil of flowers.