Studies emphasized that group of plant secondary metabolites (saponins, flavonoids and tannins) seems to present the ability to manipulate rumen fermentation lessening the CH₄ formation. The purposes of this research were to find out individual bioactive compound with anti-methanogenic activity. However, we anticipate that advances technology such as GC-MS will provide unprecedented data on the distribution of component existing in plant extracts. Leucaena leucocephala plant samples were extracted with methanol solvent using ultra sonication. Crude solvent free extract (8.58 g) was then extracted with different organic solvent with increasing polarity provided extracts of hexane (1.03 g), chloroform (0.34 g), ethyl acetate (0.48 g), butanol (0.77 g) and residual crude fractions (1.99 g) respectively. Assessment of phytoconstituents in such organic extracts was subjected to find out individual bioactive compound with the modified GC–MS (Shimadzu gas chromatography-mass spectrometer (GCMS-QP2010) method. 38 components were identified from the chromatograms of the different organic solvent extracts. Dried 0.5g of ground leucena plant, alfafa plant (positive control) and the different crude methanolic extracts with 3 different levels (125, 250, and 500 µg/mL) were tested for anti-methanogenic properties in terms of in vitro gas production and nutrient degradability. We found significant organic solvent effects for CH₄ production. Hexane extract reduces Net CH₄/OMD (P < 0.001) compared with other solvent extracts tested. Nutrient degradability, ruminal parameters and VFA production were non-significantly differed between the treatment groups. But, there was an improvement on nutrient degradability compared with Leucaena leucocephala and control plants. However, we found no doses and interaction between solvent and dose effects among the treatment groups. This study explained hexane extract from whole plant methanolic extract is effective against anti-methanogenic activity in modifying ruminal degradation of nutrients. The most active components still have to be identifying by fractionation of hexane extract.