Introduction Tachycardia-induced cardiomyopathy (TCM) is a reversible cause of heart failure with impaired left ventricle (LV) function. However, the diagnosis is difficult before treatment or control of the arrhythmia for the first time. This study was to clarify the characteristics of TCM with atrial fibrillation (AF) before AF ablation. Methods In this retrospective observational study, we observed 31 patients with paroxysmal or persistent AF who had impaired LV function without structural heart disease and who underwent catheter ablation. We defined impaired LV function as LV ejection fraction (LVEF) <50% on the initial or worst echocardiogram. After ablation, the LVEF became <60% (Group 1; n = 9) or ≧60% (Group 2; n = 22). We compared the differences in baseline characteristics between the two groups. A receiver operating curve with area under the curve (AUC) was used to evaluate the prediction efficiency. The optimal cutoff point of the AUC was at which sensitivity and specificity would be maximal. Results There were significant differences in left atrial (LA) volume (LAV) by computed tomography (CT), LAV adjusted by body surface area (LAVI) by CT, LAVI by echocardiography, and LA diameter (LAD) (p < 0.05, respectively). The AUCs were 0.859, 0.869, 0.798, and 0.750, respectively. The optimal cutoff points were 147 ml, 79 ml/m2, 37 ml/m2, and 45.8 mm, respectively. Conclusion Small LAV, LAVI by CT, LAVI by echocardiography, and LAD were predictors of LVEF improvement. Small LA volume and dimension before ablation may be useful for diagnosis of TCM with AF.