The seasonal variation in the yield and compositions of the essential oil from samples of Thymus fedtschenkoi Ronniger collected in west Iran is reported. Essential oil was obtained by hydrodistillation in a Clevenger type apparatus. Analysis was performed by GC and GC-MS. The yield in essential oil showed a maximum at the flowering stage (2.9 %) and a minimum during the seed set stage (0.7 %). The compositions also showed different patterns at different phases of the vegetative cycle. Linalool was the main component of all oils and the highest amount (83.1 %) was observed at the full flowering stage. The other main constituents identified in the oil of vegetative stage of T. fedtschenkoi were (E)-caryophyllene, α -Terpineol and α-humulene. The oil obtained from floral budding stage was dominated by α-terpineol, and α-humulene. The major components identified in the flowering stage oil were 1,8-cineole and terpinene-4-ol. Finally, the oil obtained from seed set stage was dominated by geranyl acetate and p-cymen.
