The seeds of three Eucalyptus species (Myrtaceae) (Origin: Australia), including:, Eucalyptus loxophleba Benth., Eucalyptus microtheca F.Muell. and Eucalyptus largiflorens F.Muell. were cultivated in 1993-1994 at two research stations (Shushtar and Dezful) in Khuzistan province (western south region of Iran) and a research station (Kashan) in Isfahan province (Central region of Iran). These species have good adaptability to climatic condition of this area (dry warm). The leaves of these species were collected in middle of the four seasons (spring, summer, autumn and winter) at two consecutive years, in order to find out the best harvesting time with the highest oil yield and 1,8-CINEOLE percentage. After drying the plant materials in shade, their essential oils were obtained by hydro-distillation.The oils were analyzed by GC and GC/MS devises. The results showed that the harvesting time had significant effects on oil yield and 1,8-CINEOLE content of the Eucalyptus species. The main components of the harvested oils consisted of 1, 8-CINEOLE and a-pinene. The highest oil yields of E. loxophleba Benth. in all localities were obtained from the samples collected in autumn and winter. The highest percentage of 1,8-CINEOLE for Dezful site was extracted from the winter (70.2%) and autumn (63.2%) samples, For Kashan sits, was extracted from the winter (55.5%) and spring (58.1%) samples and for Shushtar site, was extracted from the summer samples (53.6 and 49.6%). The highest oil yield of E. microtheca F.Muell. was obtained in spring from Dezful and Shushtar localities and in autumn from Kashan locality. For both Shushtar and Dezful sites, the highest percentage of 1,8-CINEOLE was obtained in winter and summer samples whereas for Kashan site, was obtained in summer samples. The highest oil yield of E. largiflorens F.Muell. was obtained in autumn and winter for Dezful and Shushtar samples and summer for Kashan samples. The highest percentage of 1,8-CINEOLE was extracted in summer and winter for Shushtar samples, in spring and winter for Dezful samples and winter for Kashan samples.