Objective To investigate whether angiotensin converting enzyme (ACE) have significant relation to permanent atrial fibrillation (Af) with mitral valvular diseases. Methods 124 consecutive lone mitral valvular disease patients who need surgery were studied. At baseline, all patients underwent a physical examination, 12lead electrocardiography and echocardiography. The plasma ACE level was measured in all patients by a radioimmunoassay technique. Patients who had permanent Af formed the Af group, and those who still kept sinus rhythm (SR) comprised the SR group. In Af group, patients were separated into two groups by the subgroup of mitral valvular disease ［mitral stenosis(MS) and mitral regurgitation(MR)］, then formed MSAf group and MRAf group. Results Af was diagnosed in 47.58% (59/124) of lone mitral valvular disease patients. Patients who had Af were older (by 6 years) than sinus rhythm patients and more frequently had a history of stroke. Mitral stenosis patients were easy to have Af (60.53% vs. 27.08%，Plt;0.05). The plasma level of ACEwas significantly higher in Af group than that in SR group (72.60 ±22.03 U/L vs. 56.40±17.96 U/L，Plt;0.05). In Af group, the ACE level in MSAf group was higher than that in MRAf group (82.92±18.75 U/L vs. 66.25±21.10 U/L，Plt;0.05). Mitral stenosis patients more frequently had a history of stroke than that of mitral regurgitation patients. Af correlated significantly with the level of ACE (r=0.089, P=0.021) and left atrial dimension (r=0.447, P=0.033). Conclusion We validated and extended the hypothesis that increasing ACE level predicted an increasing risk of Af in mitral valvular diseases. It was expressed significantly in mitral stenosis patients especially.