Eighteen natural medicinal plant essential oils and garlic oil obtained from semi-arid climate were conducted to evaluate their effects on in vitro rumen microbial fermentation and their effectiveness for decrease in vitro ruminal methane production. 50 ml of buffered rumen fluid (1:2; rumen fluid: buffer solution) were introduced in 125 ml serum bottles containing 500 mg of 80:20 alfalfa hay to concentrate as basal diet and or basal diet plus 1 μ/ml of medium of medicinal essential oils or garlic oil (6 replicates for each treatment) and incubated for 24 h at 38.7°C. Head space gas pressure of each bottle was recorded using a pressure transducer at 2, 4, 6, 8, 10, 12, 16 and 24 h of the incubation and a sample was collected to determine methane concentration. In the end of incubation pH measured and samples were collected for ammonia nitrogen concentration and dry matter (DM) and crude protein (CP) disappearance were recorded and feed fermentation efficiency (FFE = mg IDMD/ ml accumulative gas produced at 24 h post incubation) were calculated. Medicinal plant essential oils which caused an increase in FFE and decrease in gas production in-contrast with those in control were selected as the candidates for gas analysis and determine their effects on methane concentration. All essential oils resulted in a significant decrease (P < 0.05) in total gas production (except in Rosemary, Dill, Clove, Fennel, Pistachio hull and black pepper) and both IDMD and ICPD (except in Black pepper, Rosemary and Dill). Of the 19 samples tested coriander, rosemary, cinnamon, red basil, oregano 2, black pepper, cumin, caraway and dill that selected as the candidates for gas analysis and determine their effects on methane concentration increase in FFE and decrease in gas production in contrast with those in control. Increase in FFE was more noticeable for thyme, lemon pulp and cinnamon essential oils (22, 21.95 and 3.2 times more than those of control, respectively). Results indicated that
