This experiment was conducted to investigate the effect of mangosteen peel and garlic powder supplementation on intake, digestibility, ruminal fermentation and methane production in beef cattle steers fed on rice straw based diets. Four, 2 year old rumen fistulated crossbred beef cattle were randomly assigned in a 4×4 Latin square design to receive four dietary treatments; mangosteen peel pellet (Mago-pel), mangosteen peel with garlic powder pellet (Maga-lic), mangosteen peel pellet with urea and garlic powder (Maga-ulic) supplemented at 200g/h/d and non-supplemented group (control). All animals were fed with rice straw ad-libitum while concentrate was fed at 0.5% body weight (BW). It was found that supplementation of mangosteen peel and garlic with or without urea had no effect on daily feed intake (FI), digestibility of dry matter (DM), organic matter (OM) and crude protein (CP). Supplementation resulted in significantly lower overall NDF and ADF digestibility when compared with the control. Ruminal pH, concentration of ruminal NH3N and blood urea nitrogen (BUN) concentrations were not significantly different among treatments. The proportion of C2 and methane were decreased, while C3 proportion was increased by the supplementation. It was found that all supplemented treatments had no effects on Ruminococcus albus, Fibrobacter succinogenes and Ruminococcus flavefaciens populations (P>0.05), whereas total bacteria and total methanogens populations were affected by supplementation (P<0.05). Total bacteria were the highest in control group and lowest when supplemented with Mago-pel and Maga-lic. The total methanogen populations were the highest in control group and was similar with Mago-pel supplementation. Supplementation of Maga-lic and Maga-ulic resulted in lower population when compared to the control group and also with Mago-pel's supplemented groups.