Background: Stimulatory and/or inhibitory effects of angiotensin II is attributed to angiotensin receptor I. A few studies investigated the role of angiotensin receptor I on cardiovascular system in diabetic subjects. In this study, we evaluated the effects of losartan, an angiotensin II receptor I blocker, on myocardial capillary density in diabetic rats. Methods: A total number of 18 male rats were divided into 3 groups of control, diabetic, and diabetic receiving losartan. Diabetes was induced by intraperitoneal injection of streptozotocin (55 mg/kg). A daily dose of 15 mg/kg intraperitoneal losartan was injected to the treatment group. After 21 days, blood samples were taken and myocardial tissue was evaluated by immunohistochemistry. Findings: Myocardial capillary density in diabetic animals was lower than the control group (P ˂ 0.05). Losartan could not significantly change capillary density in myocardial tissue of the diabetic rats (P ˃ 0.05). Conclusion: It seems that losartan administration does not have beneficial effects on myocardial capillary density in diabetic animals. Keywords: Losartan, Angiotensin II, Capillary density, Diabetes