Olive leaf has natural bioactive compounds, mainly oleuropein, that are widely considered to have potentially beneficial effects on health. This study aimed to evaluate the effects of olive leaf extract (OLE) on the inhibition of carbohydrate digestive enzymes, and immunohistochemical study of insulin in the pancreas of in vivo streptozotocin-induced diabetic rats. Blood glucose levels, insulin, glycated hemoglobin (HbA(1c)), alpha-amylase and alpha-glucosidase activities, and an immunohistochemical study were performed at the end of the experiment. In the OLE treated group, blood glucose levels and HbA(1c) significantly decreased while insulin levels increased. Besides this, OLE treated group showed remarkable inhibitory activities on alpha-amylase and alpha-glucosidase compared with the Acarbose treated group. It was observed that OLE exhibited partial positive immunoreaction for insulin in beta-cells through immunohistochemical analysis. Considering that OLE is more tolerable for digestion system compared to acarbose, it may be a better fitoformulation for antidiabetic medications. OLE could offer an additional beneficial effect for the treatment of diabetes.