Isoquercetin is a chemical compound. It can be isolated from mangoes and from Rheum nobile, the Noble rhubarb or Sikkim rhubarb, a giant herbaceous plant native to the Himalayas. Quercetin glycosides are also present in tea. Three flavonoids from tartary buckwheat bran, namely, quercetin (Que), isoquercetin (Iso) and rutin (Rut), have been evaluated as alpha-glucosidase inhibitors by fluorescence spectroscopy and enzymatic kinetics and have also been compared with the market diabetes healer, acarbose. The results indicated that Que, Iso and Rut could bind alpha-glucosidase to form a new complex, which exhibited a strong static fluorescence quenching via nonradiation energy transfer, and an obvious blue shift of maximum fluorescence. This work would be significant for the development of more powerful antidiabetes drugs and efficacious utilization of tartary buckwheat, which has been proved as an acknowledged food in the diet of diabetic patients.