Epidemiological evidence indicates that postprandial hyperglycemia is an independent risk factor for cardiovascular disease. Improved postprandial glycemic control is promising for decreasing morbidity and mortality of cardiovascular disease in pre-diabetic and diabetic individuals. Recently, clinical trials such as the STOP-NIDDM Trial and Voglibose Ph-3 Study demonstrated that α-glucosidase inhibitor (αGI) reduces progression to type 2 diabetes from impaired glucose tolerance. Much attention has, therefore, been focused on αGI as a preventive and therapeutic agent for type 2 diabetes and its complications. Mulberry leaves have been known to prevent diabetes in Asian countries as traditional medicine. According to a previous study, mulberry leaves have strong αGI activity and its activity is caused by 1-deoxynojirimycin (DNJ), a glucose analogue. We developed an HPLC method to accurately quantify DNJ in mulberry leaves and optimized the process to achieve a DNJ-enriched (1.5%) mulberry leaf extract. We evaluated the effect of the extract on postprandial glycemic control by oral sucrose tolerance test and by a 38-day dietary trial. A dose above 0.8g of the powder (corresponding to 12mg DNJ) per, the elevation of postprandial blood glucose and secretion of insulin were suppressed significantly. Hypoglycemia, abnormal lipid profiles, or any other adverse events were not observed during and after the study period. DNJ-enriched mulberry extract may be useful in improving postprandial glycemic control in pre-diabetic or mild diabetic individuals.