The present study investigated the effects of Ginkgo biloba extract and its flavonoid fractions on α-amylase and α-glucosidase activity in vitro. Ginkgo biloba extracts and their flavonoid fraction significantly inhibited α-amylase and α-glucosidase activity in vitro. Furthermore, Ginkgo biloba extracts and their flavonoid fraction reduced the elevation of rat plasma glucose level after oral administration of various saccharinity agents. In addition, we examined the effects of the flavonoid fraction isolated from Ginkgo biloba extracts on the plasma glucose level in streptozotocin-induced diabetic rats. When flavonoid fractions were orally administered to the rats three times daily for 9 days, plasma glucose concentrations were decreased compared with those in the water treatment group. Furthermore, flavonoid fractions reduced the elevation of rat plasma glucose levels after oral administration of sucrose and glucose in streptozotocin-induced diabetic rats.