Background: We hypothesized that the post-operative aorto-mitral angle might relate to the occurrence of post-operative atrial arrhythmia (AA), including atrial fibrillation and atrial tachycardia, after mitral valve repair in patients with mitral regurgitation (MR). The purpose of the present study was to determine the effects of the post-operative aorto-mitral angle on new onset AA after mitral valve repair with mitral annuloplasty for treating MR. Methods: One-hundred seventy-two patients without any history of AA underwent mitral valve repair with mitral annuloplasty in our institution between 2008 and 2017. Patient information, including medical records and echocardiographic data, were retrospectively studied. Results: AA occurred in 15 (8.7%) patients during the follow-up period (median, 35.7 months; range, 0.5-132 months). The patients with AA had a longer cardiopulmonary bypass time and a smaller aorto-mitral angle at post-operative TTE than the others (119 ± 6 degrees vs. 125 ± 10 degrees, P = 0.003). There was no significant difference in the degree of post-operative residual MR or functional MS between the groups. In a multivariate Cox proportional hazards analysis, the longer cardiopulmonary bypass time and the smaller post-operative aorto-mitral angle were independent predictors of the occurrence of AA during the follow-up period (odds ratio per 10 minutes 1.11; 95% CI 1.02-1.22, P = 0.019: odds ratio 0.91; 95% CI 0.85-0.98, P = 0.012). Conclusion: A small aorto-mitral angle at post-operative TTE was a predictor of new onset AA after a mitral valve repair for treating MR.