The present study was conducted to evaluate the effect supplementation of sengon leaves (SL) with a condensed tannin (CT) content on the in vitro profile of cumulative gas production (GP), methane (CH₄) production, and rumen fermentation parameters. The SL containing 8.84% CT (88,4 mg per g SL) was supplemented at different levels of T0, T1, T2, T3 (0, 2, 4, and 6% CT per dry matter incubation of basal ration, respectively) during 48 h fermentation in 120 ml of serum bottles. The basal ration (BR) consisted of Brachiaria mutica grass and concentrate mix (60:40 ratio). Rumen fluid was collected from ruminally fistulated Bali cattle (Bos sondaicus). The profile of cumulative gas and methane production was fitted using y= a+ b (1 – e^-ct). The profile of cumulative gas and methane production was lower at T3 and T2 than at T1 and T0. Methane gas production stabilized after 24 h of incubation for T3 and T2. Meanwhile, the profile of methane GP was stabilized after 36 h for T1 (2% CT) and after 48 h for T0 (0% CT) (control). The fermentation parameters measured in this study showed that increased supplementation with CT-SL significantly (P<0.05) decreased the in vitro dry matter digestibility (IVDMD), in vitro organic matter digestibility (IVOMD), ammonia (N-NH₃), total volatile fatty acid (TVFA), total gas (TG, ml), total methane (ml) and methane percentage (CH₄ per TG). The study concluded that supplementation of CT-SL at T2 (4% CT equal to 0.09 g SL/0.2 g BR, or equal to supplementation of 45% SL per BR (DM basis) was more effective in controlling methane production and was still favorable in IVDMD, IVOMD, N-NH₃, and TVFA to maintain ruminal microbial activities and ruminant needs. The results suggest that evaluation under in vivo conditions is needed.