Groat rice is the side product of rice milling process and potentially producing α-glucosidase. The enzyme has an important role in starch degradation process by breaking α-glycosidic bods to form glucose. This research aimed to understand the optimum activity of the purified α-glucosidase. The research stages are isolation, purification, characterization of substrate concentration, pH level, incubation time, temperature, and effect of metal ion, and dinitrosalicylic acid (DNS) method to determine the activity of α-glucosidase. The result showed the specific activity of α-glucosidase crude extract at 0.003 IU/mg. Fractionation of the highest specific activity on fraction 4 revealed at 0.042 IU/mg and constantly increasing on the dialysis stage at 1.121 IU/mg. α-glucosidase reached optimum activity at substrate concentration 2%, pH of 7, incubation time of 40 minutes, at 37°C, with metal ion Mn2+, Co2+, Na+, Mg2+, Ca2+ and K+ as activators and Zn2+ as inhibitor. α-glucosidase could be used to find analog compounds as the cure of type 2 diabetes mellitus.