Objective This study aimed to evaluate the effect of Corni Fructus water extract on streptozotocin-induced diabetes in rats. Methods Five-week-old, male Sprague-Dawley rats were divided into four groups, such as i) normal group, ii) STZ-control group, iii) 300 mg/kg body weight Corni Fructus (CF) water extract-administrated group and iv) 250 mg/kg body weight metformin treatment group, as a positive control for 4 weeks. Rats in which diabetic was induced by intraperitonal injections with STZ (30 mg/kg body weight) for 3 consecutive days. Fasting glucose, triglyceride, total cholesterol and HDL-cholesterol were measured in sera of rats. Total volume of urine and urinary creatinine were also measured using commercially available kits. Histopathological examination of kidney was performed. Results There were no differences in body and kidney weight between the STZ-control and CF treated groups. However, serum glucose, triglyceride, total cholesterols levels were significantly decreased and HDL-cholesterol levels were significantly increased in CF extract-administrated groups compared with those of STZ-control group. Histopathological analysis of kidney revealed decreased morphological changes of glomerulus after the administration of CF extract. Conclusion These results suggested that Corni Fructus has an anti-diabetic effect in STZ-induced diabetic rats via decreasing the serum levels of glucose, triglyceride, total cholesterol and HDL-cholesterols in serum, and suppressing the morphological changes of kidney.