This study aimed to investigate the effect of tannin extract from mangosteen peel (MANGTAN) in soybean meal on in vitro gas production kinetics and digestibility. Completely randomized design (CRD) was used as experimental design with eight treatments consisted of 5, 10, 15, 20, 25, 30, 35 and 40 ml of MANGTAN/ 100 g of soybean meal. It was found that MANGTAN influenced the gas production kinetics and cumulative gas production at 120h of incubation which was found the lowest gas volume at level 15 ml of MANGTAN/100 gram of soybean meal (P < 0.05). In vitro dry matter and organic matter digestibility at 12 and 24 hr. were higher in MANGTAN supplementation at 15 ml of MANGTAN/100 g of soybean meal when compared to the other groups except treatment at level of 2 0 ml of MANGTAN/100 gram of soybean (P < 0.01). Based on the results, it could be summarized that MANGTAN supplementation could reduce gas production kinetics but increase in vitro digestibility. However, this research should be studied more that is MANGTAN supplementation can protect protein in the rumen. Therefore, more research
should be done especially, study on protein digestion.