The present study was designed to clarify the antidiabetic activity and mechanism of Dioscorea rhizome in diabetic db/db mice. Mice were administered Dioscorea rhizome and rosiglitazone orally for 7 weeks and the effects of these compounds on fasting blood glucose, glucose tolerance and intestinal disaccharidase activity in db/db mice were evaluated. The fasting serum glucose of the D. rhizome treated group was reduced when compared with that of the db/db control group. In addition, the disaccharidase activities in homogenates of the proximal, middle and distal segment of the small intestine were significantly decreased response to D. rhizome treatment, especially in the middle segment. These results suggest that D. rhizome decreases blood glucose via a decrease in the activity of disaccharidase in the mucosa of the middle region of the small intestine in db/db mice.