This study was aimed  to evaluate  Albizia falcataria with condensed tannin content in the diets on gas production and ruminal fermentation parameters. A completely randomized design (CRD) with fours treatment and five replications had applied in this study. The treatment in this study was fours combined of Brachiaria mutica and Albizia falcataria namely; R0: 60% Brachiaria mutica (BM) + 0% Albizia falcataria (AF) + 40% concentrate (CON), R1: 50% BM + 10% AF + 40% CON, R2: 40% BM + 20% AF + 40% CON and R3: 30% BM + 30% AF + 40% CON. The concentrate formulated using feed ingredients consists of 58% rice bran, 15% soybean cake meal, 30% corn, 1% salt, and 1% multi-vitamin minerals. The Diets samples were  incubated at 39^oC. The volume of gas and CH₄ were recorded manually at 3, 6, 9, 12, 24, and 48 h post-incubation. Dry matter digestibility (DMD), organic matter digestibility (OMD), pH, N-ammonia (N-NH₃), protozoa count, and microbial protein production (MP) were measured at 48 h post-incubation. The study showed that increased use of AF decreases (P<0.05) total gas, CH₄, percentage of CH₄, and increases the potency reduction of CH₄. There is a negative correlation (R²= 0.81) between tannin condensed levels and CH₄ production. Concentration N-NH₃ and protozoa count was significantly (P<0.05) lower with the use of AF in treatment of R1, R2, R3 than R0 (without AF). As well as significantly (P<0.05) increased MP production. The study had concluded that the use of AF in the diet of R3 (30% BM : 30% AF) as a forage source reduced methane gas and N-NH₃ by 62.31% and 25.73%, increasing MP, and without retarded the activity of rumen microbes.