Background: It is known that angiotensin receptor blockers have electrical-stabilizing effects by preventing atrial remodeling. Experiments, which demonstrated improvements of atrial remodeling by angiotensin receptor blockers in rapid atrial pacing models, are relatively common. Even though atrial fibrillation is frequent in ischemic heart failure clinically, experiments to demonstrate angiotensin receptor blocker's effects on atrial remodeling in a heart failure model are rare. Materials and Methods: Heart failure model and sham-operated group were made in 28 Sprague-Dawley male rats with about 260g of weight. Ischemic heart failure models were obtained after making myocardial infraction by ligation of proximal part of a left anterior descending coronary artery. In losartan group, 30 mg/kg of losartan was administrated in a day for 4 weeks. Echocardiography was performed to measure left ventricle ejection fraction(LVEF) and left atrial diameter(LAD) at baseline and 2 weeks after the operation. Finally 4 weeks later, formalin perfusion fixation was done in all rats. Each heart was quickly excised for histologic and immunohistochemical evaluation. Results: Group was divided into sham-control group, sham-losartan group, heart failure-control group, heart failure-losartan group. We could maintain 5 rats in each group for 4 weeks after operation. LVEF increased from 55.4 짹 4.03 % to 59.9 짹 2.47 % in Sham operated-control group. LVEF decreased from 58.3 짹 2.40 % to 57.0 짹 1.20 % in Sham operated-losartan group. LVEF decreased from 68.9 짹 11.1 % to 45.1 짹 6.22 % in heart failure-control group. LVEF decreased from 61.8 짹 6.72 % to 48.8 짹 12.45 % in heart failure-losartan group. LAD increased from 3.8 짹 0.42 mm to 5.6 짹 0.78 mm in Sham operated-control group. LAD decreased from 4.2 짹 0.00 mm to 4.1 짹 0.57 mm in Sham operated-losartan group. LAD increased from 4.3 짹 0.14 mm to 8.7 짹 0.49 mm in heart failure-control group. LAD increased from 4.5 짹 0.57 mm to 6.6 짹 0.71 mm in heart failure-losartan group. Less fibrosis was identified in heart failure-losartan group than in heart failure-control group through Masson's trichrome stain. More expression of connexin 43 protein was identified in heart failure-control group than in heart failure-losartan group. Conclusion: Angiotensin II receptor blocker, losartan, has preventive effects on structural and histological atrial remodeling in ischemic heart failure model of rats. The protection against atrial remodeling might play an important role in prevent atrial fibrillation. However, it might be necessary to rule out the probability of secondary effects from improvement of left ventricle heart failure by an angiotensin II receptor blocker.