Studies were conducted to investigate the composition of essential oil of wormwood (Artemisia absinthium L.) growing wild in Iran. The wormwood aerial parts were harvested in full blooming time from an area between Deylaman and Asiabar villages, at Alborz altitudes in Guilan province in September 2005. Aerial parts were dried at shade (room temperature) for several days and their essential oil was extracted by hydrodistillation method in a Clevenger apparatus and analyzed by GC/MS. Results showed that essential oil yield was 1.3%. Twenty eight components representing 93.3% of the oil were identified, which were mostly monoterpenes. beta-pinene and beta-thujone were the main components, which their contents were 23.8 and 18.6% respectively. The largest part of the essential oil was formed by hydrocarbon monoterpenes (47.8%). The results proved that chemotype of the studied wormwood essential oil was specific and different from other wormwood essential oil chemotypes, which have been reported so far.