Diabetes mellitus, a metabolic chronic disorder characterized by high blood sugar level, is one of the most serious and common diseases in the world, leading to major complications such as diabetic neuropathy, retinopathy and cardiovascular diseases. The World Health Organization classified into two types of diabetes, Type 1 diabetes which is insulin-dependent and Type 2 diabetes, non-insulin dependent diabetes. The widely used treatment for non-insulin dependent diabetic mellitus is to disturb hydrolytic cleavage of carbohydrate and retard glucose absorption. To avoid hyperglycemia, inhibition of digestive enzymes such as α-glucosidase and α-amylase was used as therapeutic approach for type 2 diabetes. Alpha-glucosidase inhibitors (AGI) such as acarbose have been used to lower blood glucose level by suppressing the activity of α-glucosidase and α-amylase in digestive organs. But microbial synthetic AGIs have side effects such as gas, diarrhea, nausea, and cramps. Therefore, many plants having natural AGIs without these kind of side effects have been studied. Particularly, flavonoids, a large class of at least 6,000 phenolic compounds found in various plants, were reported as candidate substances which control glucose level in blood. In this review, studies on flavonoids as inhibitors of α-glucosidase and α-amylase in plants, the mechanisms of candidate substances, and the relationship between its structure and AGI activity are discussed.