Rice straw as ruminant feed has low digestibility makes ruminants difficult to digest. Since ammoniated rice straw can increase its digestibility, it is necessary to supplement it with Gliricidia sepium. This research aimed to determine the effect of Gliricidia sepium supplementation on decreasing methane gas production and improving nutrient digestibility in complete ration with ammoniated rice straw. The treatments of this research were: A = 40% ammoniated rice straw + 60% concentrate, B = 40% ammoniated rice straw + 50% concentrate + 10% Gliricidia sepium, C = 40% ammoniated rice straw + 40% concentrate + 20% Gliricidia sepium, D = 40% ammoniated rice straw + 30% concentrate + 30% Gliricidia sepium. This research showed supplementation of G. sepium significantly decreased methane gas production from 27.22 mM to 13.13 mM and the number of protozoa from 6.3 x 105 cell/ml rumen fluid to 4.7 x 105 cell/ml rumen fluid. Gliricidia sepium supplementation increased digestibility significantly (P < 0.05). Digestibility of dry matter increased from 58.83% to 68.54% and digestibility of organic matter also increased from 59.50% to 69.50%. Total VFA (Volatile Fatty Acid) concentration did not differ significantly (P > 0.05) among treatments. Higher levels of G. sepium supplementation increased N-NH3 concentration from 7.33 mM to 10.50 mM and microbial protein synthesis from 74.33 mg/100ml rumen fluid to 108.25 mg/100ml rumen fluid. The treatment had a significant effect (P <0.05) on propionate production and the ratio of acetate: propionate. It can be concluded that 30% of Gliricidia sepium supplementation decreased methane gas production and the number of protozoa. Besides, 30% of Gliricidia sepium supplementation increased digestibility and rumen fermentation.