Possible effects of blackberry leaf aqueous extract on blood glucose and other serum components in normal and STZ –induced diabetic rats were investigated. Blackberry leaves extracts were prepared by maceration method and gavaged for 28 consecutive days to the animals. Rats with blood glucose level greater than 200 mg/dl, after injection of 70 mg/kg STZ, were enrolled in the study. Blood glucose levels in rats, at zero time, 1 and 3 hours after gavages, and each week was measured. In the 14th and 28th days, serum components, and on the last day, the weight of liver, kidney and heart were measured. Within four weeks, blood glucose levels in the group receiving the extract had a significant decreasing trend. In the group receiving the extract, serum lipid parameters and liver function tests, dropped from second to fourth week, compared with positive control group, was significant. This study showed that oral administration of aqueous extract of Blackberry leaves can have the blood sugar-lowering effects and reduction of serum lipids and liver function tests.