The aim of the current study was the screening the activity of 23 medicinal plants for decreasing the in Vitro ruminal methane production, as well to and assess sheep ruminal in vitro fermentation parameters. 57.3 mg of each medicinal plants along with 142.7 mg control diet were weighted in 120 ml wheatons in 4 replicates. The samples were incubated using ruminal fluid obtained from 3 castrated male Ghezel sheep with 61.8 kg of body weight and 4 years of age. After 24 h of incubation, total Gas Production (GP), methane production, ammonia content as well as Volatile Fatty Acids (VFA) were assessed. The obtained data were statistically analyzed using a completely randomized design. The results showed that GP from the incubation of Allium hertifolium, Tribulus terrestris L and Foetida ferula-assa increased in comparison with the control treatment (p<0.05). The molar proportion of methane in GP was decline by 20% in treatments containing Allium hertifolium, Elettaria cardamomum Maton, Thymus vulgaris, Nigella sativa, Rosmarinus officinalis, Rheum palmatum, Chamaemelum nobile, Sygium aromaticum and Thea sinensis in comparison with the control treatment (p<0.05). The concentrations of total VFA in treatments containing Zingiber officinale, Nigella sativa, Chamaemelum nobile, Trachyspermum copticum and Thea sinensis were higher than that in control (p<0.05). On the other hand, treatments containing Allium hertifolium, Foetida ferula-assa, Alpinia officinarum, Elettaria cardamomum Maton, Achillea milhemsii, Glycyrrhiza glabra, Rosmarinus officinalis, Pimpinella anisum, Chamaemelum nobile, Hyssopus officinalis and Fumaria officinalis Lan contained lower levels of ammonia concentrations than in the control diet (p<0.05). The results indicated that the treatment containing Allium hertifolium lowered the methane production and ammonia concentration without any negative effect observed on the total gas production.