Four swamp buffaloes (Bubalus Bubalis) with average body weight of 238±27 kg were randomly assigned to receive four dietary treatments according to a 4 x 4 Latin square design. The treatments were supplemented with Leucaena leucocephala leaves (LLP) and were as follows: Treatment (T1) = LLP supplementation at 0 g/h/d (control); T2 = LLP supplementation at 150 g/ h/d; T3= LLP supplementation at 300 g/ h/d; T4= LLP supplementation at 450 g/ h /d. All animals were fed with concentrate (14.2 % CP, 76 % TDN) at 0.5% BW and offered with rice straw ad libitum. It was found that dry matter intake and ruminal NH3-N (16.2 mg/dl) were increased at highest LLP supplementation, but no effect on rumen pH. Apparent digestibilities of DM, OM, CP, NDF and ADF were significantly increased (P<0.05) when increasing LLP supplementation and was highest at 450 g/h/d. Ruminal NH3-N concentration was significantly different among treatments (P<0.05), and was highest when buffaloes were fed at 450 g/ h /d (16.2 mg/dl). Blood urea-nitrogen (BUN) concentrations were not significantly different (P>0.05) among treatments but tended to be increased with enhancing levels of LLP supplementation (9.0-11.6 mg/dl), respectively. Total volatile fatty acids (TVFA) and proportion of propionic acid (C3) were differed among treatments and ranged from 88.6 to 100.8 mmol/l and 23.4 to 27.7 mmol/l, respectively. The concentrations of acetic acid (C2) and butyric acid (C4) were found decreased when increasing level of LLP from 300 to 450 g/ h /d while the ratio of acetic acid to propionic acid were decreased when supplementation of LLP from 300- 450 kg/ h /d. Based on this study, it could be concluded that supplementation of LLP at 450 g/ h /d with rice straw can improve voluntary intake, digestibility and rumen fermentation parameters in swamp buffaloes.