Costus pictus D. Don, commonly known as ‘insulin plant’ is a member of Zingiberacea family and is used as a munching dietary supplement for the treatment of diabetes in Southern India. The present study was carried out to evaluate the antidiabetic effect of Costus pictus leaves in normal and streptozotocin-induced diabetic rats. The oral feeding of aqueous leaf solution of this plant in diabetic rats for 28 days at a dosage of 2gm/kg body weight exhibited a significant (p<0.001) reduction in fasting blood glucose level and a remarkable increase in serum insulin level. There was a significant reduction (p<0.001) in serum parameters like SGOT, SGPT, lipids, triglycerides, total cholesterol, urea, TBARS, and albumin in diabetic rats treated with leaf solution. The body weight of diabetic rats was restored to normal state when treated with the C. pictus. Morphometric analysis of C. pictus-treated rat pancreatic islets showed a significant (p<0.001) increase in the number and area of islets when compared with normal and diabetic control rats. Histopathology studies in liver and kidney of diabetic rats treated with aqueous solution did not show any marked difference from normal which revealed the non-toxic effect of this plant. Estimation of trace elements using particle induced X-ray emission analysis in the leaf was also determined to find the antidiabetic potential elements in this plant. Based on the above results it is evident that the leaves of C. pictus have antidiabetic effect and must be considered as a potential candidate for future studies on diabetes mellitus.