Background: Reactive oxygen species caused by hyperglycemia in diabetes mellitus (DM) are considered a compelling cause of male subfertility and infertility. Insulin or other oral anti-diabetic drugs are not effective in preventing diabetic complications. Rosemary has a hypoglycemic effect with antioxidant activity and a protective role against testicular toxicity. Aim: evaluating the role of rosemary oil in modifying the diabetogenic effect on rat testes. Materials and Methods: Six groups of 30 rats, each with five rats, were set up. The groups were +ve Cont, rosemary oil, DM, DM + Insulin, DM + Rosemary oil, and DM + Insulin + Rosemary oil. In the diabetic groups, DM was induced with streptozotocin (STZ) (40 mg/kg, I.P.) once. Rosemary oil was administrated (50 mg/kg, I.P) daily in the rosemary oil groups. The insulin groups were treated with subcutaneous insulin (14 IU/kg/day). Treatments in all groups were given for four weeks after confirmation of DM. Results: body weight, blood glucose levels, sperm parameters, hematoxylin & eosin toluidine blue, and immunostaining sections with morphometric measurements and catalase levels presented improvement in rosemary, insulin, and combined treated groups in an uprising manner. Conclusion: Combined Rosemary oil and insulin ameliorated testicular complications resulting from STZ-induced DM in adult male albino rats.