Background: Buni (Antidesma bunius [L.] Spreng) has been used as a traditional antidiabetic agent in Asia. Objective: The mechanism of antidiabetic properties was studied in this study by determine its α-glucosidase inhibitory activity. Method: Inhibition of α-glucosidase was performed in all fraction of Buni stem bark with acarbose and miglitol as standards. The half maximal inhibitory concentration (IC50) value of acarbose and miglitol was 5.75 and 59.76 μg/mL respectively while ethyl acetate (EtOAc) fraction was the most active fraction with IC50of 19.33 μg/mL. Three isolates (B1, B2, and B3) were found in the EtOAc fraction and elucidated by infrared, 1hydrogen-nuclear magnetic resonance, 13carbon-nuclear magnetic resonance, and two-dimensional nuclear magnetic resonance. Result: The chemical structures of the isolates were identified by the spectrum then compared with literature which concluded that B1 is friedelin, B2 is β-sitosterol, and B3 is betulinic acid. Inhibition of the α-glucosidase assay showed IC50values of B1, B2, and B3 were 19.51, 49.85, and 18.49 μg/mL, respectively.