Recently, bioactive components of plants and plant parts of most trees and browse species have been used as rumen modifiers to reduce methane gas production in ruminants, thereby reducing their contribution to the implicated greenhouse effect. This study, therefore, evaluated the probable use of Chromolaena odorata (L.) R.M.King & H.Rob. (Siam weed) leaves as rumen modifier in West African Dwarf Bucks. Fresh C. odorata leaves were harvested, air dried for 3 weeks, milled using a 2 mm sieve size and bagged for both proximate and phytochemical analysis. Concentrate diets were formulated with C. odorata leaf meal included in the diet at 0, 2, 4 and 6% of the whole diet. Rumen fluids were collected from West African Dwarf (WAD) bucks (averaged 25 kg) using suction tube and assigned to the 4 experimental diets in a completely randomized design (CRD). The incubation of inocula was performed for 96 hours with 12 replicates per treatment in a single run and data obtained were subjected to one-way analysis of variance and the mean values were compared with Tukey’s Test. The results indicated that C. odorata had 969.0mg/kg dry matter, 17.51 % crude protein, 20.43% crude fibre, 52.16% nitrogen free extract, 1.99% saponin, 2.57% tannin, 1.08% flavonoid and 1.26% alkaloid. Addition of 2 and 4% C. odorata leaves to the diets resulted in increased (P < 0.05) in vitro gas production while C. odorata at 2% reduced (P < 0.05) the methane gas (%) estimate. In vitro organic and dry matter digestibility, total digestible substrates and short chain fatty acids were increased (P < 0.05) by the addition of C. odorata leaves to the diets. This study concluded that the use of C. odorata as an additive at 2 and 4% inclusion increased total gas output. However, 2% inclusion is beneficial as it reduces the net methane production while maintaining higher gas production and digestibility.