Objective: To study the pathological basis of right atrial fibrillation in rheumatic heart disease (RHD) patients with atrial fibrillation (AF). Methods: Twenty-nine patients with mitral valve replacement of RHD were divided into AF group (n = 13) and sinus rhythm group (SN group) (n = 16). There was no significant statistical difference in clinical factors between the 2 groups. During the operation of valve replacement , the samples of right atrial appendages were taken and the qualitative and quantitative study were made by light microscopy and electron microscopy. Results: (1) Light microscope: The interstitial fibrosis and the arrangement of myocardium was more disordered in AF group than that in SN group. However, no statistic difference was found in interstitial fibrosis and cellar hypertrophy degree between the 2 groups. (2) Electron microscope: Mitochondrial crosta broke and dissolved obviously in AF group. The mitochondrial volume in AF group was smaller than that in SN group. Volume density, a Objective: To study the pathological basis of right atrial fibrillation in rheumatic heart disease (RHD) patients with atrial fibrillation (AF). Methods: Twenty-nine patients with mitral valve replacement of RHD were divided into AF group (n = 13) and sinus There was no significant statistical difference in clinical factors between the 2 groups. During the operation of valve replacement, the samples of right atrial appendages were taken and the qualitative and quantitative study were made by Light microscope: The interstitial fibrosis and the arrangement of myocardium was more disordered in AF group than that in SN group. However, no statistic difference was found in interstitial fibrosis and cellar hypertrophy degree between the 2 groups. (2) Electron microscope: Mitochondrial crosta broke and dissolved obviously in AF group. The mitochondrial volume in AF group was smaller than that in SN group. Volume density, a