The aim of the present study was to evaluated effect of some Iranian native essential oils at different doseson in vitro ruminal methane production of a high concentratediet. Experimentaldiet that used for batch cultures was a mix of concentrate: alfalfa hay (80:20, based on DM) which was ground to pass from I-mm screen. Approximately, 500 mg of the diet alone (as control) or plus cinnamon (Ci), Oregano (Or), Pistachio hull (PH), Coriander (Cor) and Thyme (Th) essential oils (35, 70, 140and 280 Ill/Iof culture fluid) were placed into a 125 ml serum bottles (n=6) containing 50 ml of buffered rumen fluid (ratio of buffer to rumen fluid was 2: I), then bottles were placed in water bath for 24 h at 38.5 qc. Rumen fluid was obtained from three adult ruminally fistulated sheep, before the morning feeding. In the end of incubation gas production of each bottles was recorded and a sample of the gas was collected into a 10ml evacuated tubes. Methane content of the produced gas was determined using gas chromatography procedure. Data were statistically analyzed using SAS (V. 9/1) and differences between each treatment and control were tested using Dunnett's test (P<0.05). The results indicate that all ofthe essential oils (except Cor) were effective (P<0.05) in decrease of methane produced compared with those of the control. Relative to the control increasing concentration of essential oil (except for Cor) resulted to decrease in methane production. Results of the present study concluded that TH, Ci and PH essential oils have a high potential to decrease ruminal methane production

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