This study investigated the in vivo insulin function of Polygonatum odoratum in normal and diabetic male Sprague-Dawley rats. Diabetes mellitus was induced by an i.p. injection of streptozotocin. Normal and diabetic rats were assigned to the diet groups of the control basal diet and Polygonatum odoratum diet. The animals were fed the diet and water ad libitum for 15 days. Initial and final body weights, total food intake and serum glucose and insulin levels were measured. An insulin suppression test was performed to elucidate the insulin function in the peripheral tissues. The results showed that the final serum glucose levels significantly decreased in the diabetic rats on the Polygonatum odoratum diet compared with the diabetic rats on the control diet. The final serum insulin levels were increased in the diabetic rats on the Polygonatum odoratum diet compared with the diabetic rats on the control diet. The in vivo function of the insulin increased in the diabetic rats on the Polygonatum odoratum compared with the diabetic rats on the control diet. These data indicate that Polygonatum odoratum may be beneficial in improving the in vivo insulin function in streptozotocin-induced diabetic rats.