Back ground: Diabetes mellitus (DM) is a major health problem worldwide in recent time. Asia and Africa are the most viable areas where the disease is feared to raise 2–3 folds. Many herbal products have been recommended for the treatment of DM in ancient literature of Ayurveda in India and other worldwide. Material and method: Thirty male adult albino rats were used to investigate the effect of Catharanthus rosea (C.roseus) on diabetic rats. Rats were divided into three equal groups, control, diabetic non treated and diabetic C.roseus treated groups. After thirty days of treatment all rats of each group were sacrificed. The body weight of each rat was determined at the beginning and the end of each period. Blood glucose, serum insulin, lipid and protein profiles, liver and kidney functions, blood picture and liver glycogen were determined for each rat at the end of each period. Pancreatic samples were obtained and processed for microscopic and quantitative evaluation after staining the prepared sections with heamatoxylin and eosin as well as special stain for demonstration of the different pancreatic cells in the islets of Langerhans. Results: The obtained results showed that the diabetic rats were diagnosed by laboratory assessment to body weight loss, hyperglycemia, and hypoinsulinemia, significant increase in liver and kidney functions, lipid and protein profiles and decreased liver glycogen content. While, C.roseus treatment led to a significant improvement in these parameters except liver function. Microscopically there was definite vaculation, degeneration, karyolysis and pyknosis of beta pancreatic cells in the diabetic group, while other pancreatic cells were not affected (alpha and delta cells). The use of C.roseus treatment of this study greatly improves such cellular changes. Conclusion: It was recommended that the use of the water extract of C. roseus levies as a hypoglycemic agent may offer a new hope to the diabetics in future. It's well recommended to use variable doses and different periods of treatment to evaluate the best dose and period.