Cinnamomum zeylanicum (Cinnamon) belongs to the family Lauraceae and is used to cure bronchitis, asthma, diarrhoea, nausea and vomiting, flatulence, fever, headaches, bad breath and toothaches. Its powder is a very good appetizer and improves digestion, while its oil “cinnamon oil” has been shown to exhibit antibacterial, antifungal, antispasmodic and antiinfectious properties. The aim of the current study was to estimate the plant secondary metabolites in aqueous methanol extract of C. zeylanicum and their effect on in vitro methanogenesis and rumen fermentation on adding different level (1, 2 and 3ml) of aqueous methanol extract of C. zeylanicum bark powder on three different sorghum based diets i.e. low, medium and high fiber diets (LFD, MDF and HFD). Aqueous methanol extract contain the 5.04%, 0.03%, 0.11% and 0.05% total sugar, saponins, total protein, and total tannins on dry matter basis respectively. Evaluation of aqueous methanol extract of C. zeylanicum bark powder was carried out using in vitro gas production technique. Results showed the maximum methane reduction (45.37% in term of mM/gDDM) in HFD at 3 ml of extract. In case of HFD diet and 1 ml level of extract TVFA, propionate and butyrate production were increased 59.13, 27.81 and 56% respectively.