Tannins is the novel feed strategy that can mitigate enteric methane emissions. This study characterized the response of in vitro rumen methanogenesis and the ruminal fermentation of Mango tree by- products (deciduous mango leaves (DML) and Waste Trimmings of Mango Trees (WTM)) rich in tannins. Results of Anti-nutritional factor founded that both of DML and WTM showed higher value of tannins (2.0 and 2.4 mg/100g) respectively. The highest significant GP, gas production structure fiber (GPSF) and gas production non structure fiber (GPSNF) were observed in DML and recorded 20.44, 10.04 and 34.49 (ml/g DM), respectively. The First laboratory experimental study results exhibited highly significant differences in ruminal fermentation characteristics across control diet and diets with DML. The highest significant GP, GPSF and GPSNF were observed in control diet which recorded 49.19 (ml/200mg DM), 14.26 (ml/g DM), and 43.45 (ml/g DM), respectively. Also, ME (MJ/Kg DM), NE, DCPL (g/day), DOMI (g/day), Microbial protein (g/DOM kg), and GED (g/Kg DDM) were significantly higher with control diet. The obtained results of the second laboratory experiment showed significant over than the control diet in GPSF, GPNSF, SCFA, energy content, DOM, ME (MJ/Kg DM) , NE, DCPL (g/day), DOMI (g/day), microbial protein (g/DOM kg), and GED (g/Kg DDM) in comparison with 5% and 10% from WTM as a replacement of wheat bran. The results of the in vitro methanogenesis study found that Mango tree by- products containing tannins have the ability to inhibit methanogenesis