Objectives: The present study investigated the protective effect of Wattakaka (W.) volubilis leaf extract against streptozotocin (STZ)-induced diabetes in rats. Methods: Male Wistar rats were divided into five groups (with six rats in each group) and were fed ad libitum. The rats were fasted for sixteen hours before diabetes was induced by injecting a single dose of 90 mg/kg body weight of STZ in 0.9-percent normal saline through an intraperitoneal route. The five groups were as follows: Group 1: normal control (saline-treated), Group 2: untreated diabetic rats, Groups 3 and 4: diabetic rats treated orally with petroleum ether cold maceration extract (PEME) of W. volubilis (50 and 100 mg/kg body weight), and Group 5: diabetic rats treated orally with metformin (250 mg/kg body weight). All rats received treatment for 21 days. For the STZ-induced diabetic rats, the blood-glucose, ${alpha}$-amylase, total protein and alanine transaminase (ALT) levels were measured on days 7, 14 and 21 of the treatment with PEME of W. volubilis and the treatment with metformin. Histopathological changes in the liver were examined with hematoxylin-eosin staining. Morphological changes in the liver were also examined with glutaraldehyde fixation. Results: The treatments with PEME of W. volubilis and with metformin in experimental rats by oral injections for 21 days produced reductions in the levels of serum biochemical markers. Histopathology and scanning electron microscopy results showed that the administrations of PEME of W. volubilis and of metformin suppressed the generation of abnormal liver cells in the STZ-treated rats. Conclusion: These results suggest that both PEME of W. volubilis and metformin have a protective effect against STZ-induced diabetes.