This study aims to analyze the effect of supplementing maja leaves in ruminant feed on the balance between nutrient utilization and environmental impact, particularly in reducing rumen protein degradation and suppressing methane gas production. The study was conducted in vitro using a completely randomized design with four treatments (maja leaf levels of 0, 10, 20, and 30 g/100 g of feed) and four replicates, analyzed using One-Way ANOVA, followed by the Duncan test. The results showed that adding maja leaves at 30 g/100 g, with a tannin content of 2.38%, had no significant effect on NH3 concentration, pH, and total protozoan population. However, it significantly influenced gas production, CH4 production, and undegraded rumen protein. Both gas production and CH4 output decreased with the addition of Maja leaf levels in the ration. The highest level of undegraded rumen protein, 34.8%, was achieved with the supplementation of 20 g/100 g of maja leaves, while the lowest was found in the treatment without added maja leaves. It was concluded that supplementing maja leaves at a level of 30% did not affect rumen fermentation characteristics. The lowest total gas production and CH4 were found in the CC30 treatment, while the percentage of undegraded protein that could bypass rumen degradation, at 34.8% in the CC20 treatment, was the highest among all treatments.