ABSTRACT Diabetes mellitus is a syndrome that reaches more than 382 million people worldwide. It interferes with the metabolism of carbohydrates, causing chronic hyperglycemia. The objective of this study was to evaluate the effect of the Copaifera duckei, Dwyer, Fabaceae, oleoresin on streptozotocin-induced (STZ) diabetic rats. This study was based on the induction of diabetes mellitus by streptozotocin (55 mg/kg, i.p.) in Wistar rats and treated with doses of C. duckei oleoresin (250 and 500 mg/kg, p.o.). Subsequently, the clinical, biochemical and histopathological of the pancreas parameters were evaluated. Gas chromatographic analysis indicated that β-bisabolene (22.29%), β-caryophyllene (21.25%) and α-farnesene (15.58%) sesquiterpenes were the major components of the C. duckei oleoresin. In streptozotocin-induced diabetes mellitus, it was possible to observe that the C. duckei oleoresin treatment had a significant effect (p < 0.001) on the clinical parameters, and that there was a positive improvement. This was attenuated by the urea, creatinine, and transaminases alterations (p < 0.001) observed in animals with diabetes mellitus, as well as the significantly reduced (p < 0.001) values of total cholesterol, triacylglycerides, and glucose. In the histopathological analyses of the pancreas, it was observed that the C. duckei oleoresin was able to restore β-cells and to significantly increase the quantity and diameter of the Langerhans islets (p < 0.05), when compared to the diabetic group. The treatment with C. duckei oleoresin, employed under the conditions of this study, presented antidiabetic activity and can improve the complications found in this syndrome.