It is well known that there is close relation of the left atrial pathology to atrial fibrillation(AF) in patients with mitral valvular disease. To evaluate such relation is existed or not in the right atrium, the right atrial wall biopsy was performed in 31 patients with mitral valvular disease during valve surgery. The specimens obtained were interpretated by the pathologist who had no information about the patients. The specimens were graded according to severity of morphological change. In Grade I, the atrial myocardium is degenerated and has moderate fibrosis in it. Grade II shows the atrial myocardial layer of the atrium and severe degenerated with pre and postoperative heart rhythm. Four patients had Grade I change and 3 patients among them had pre and postoperative AF. Eleven patients had Grade III changes and 9 among them had pre and postoperative AF. The results show there is strong tendency that the patients with pre and postopeative AF had Grade III change in the right atrium and the relation of the right atrial pathology to AF is same as that of the left atrium.