This study investigated the effect of Corni Fructus (Cornus officinalis Sieb. et Zucc.) extract on blood glucose and insulin resistance in db/db mice. Seven weeks old male mice were divided into normal control group (NC), diabetic control group (DC) and Corni Fructus treated diabetic group (DCF). Over an 8-week experimental period, Corni Fructus extract was administered orally at 500 mg/kg BW/day. Corni Fructus inhibited increase in blood glucose level during the OGTT (oral glucose tolerance test). At 8 weeks after beginning of the experiment, blood glucose level in the DCF group was significantly lower (p<0.01) than the DC group. Final fasting serum glucose and triglyceride in the DCF group were significantly lower (p<0.05) than the DC group by 32% and 41% respectively. Serum insulin did not differ among the NC, DC and DCF groups. The mRNA expression of adiponectin, GLUT 4 and PPAR-$gamma$ in adipose tissue in the DC group were significantly lower than the NC group and they were higher in the DCF group than the DC group by 76%, 130% (p<0.05) and 43%, respectively. In conclusion, these results indicated that Corni Fructus would have antidiabetic effects via improving insulin resistance in favor of higher glucose utilization and reducing blood glucose level in db/db mice.