Background: The left atrial (LA) substrate has been suggested the determinant of occurrence and maintenance of atrial fibrillation (AF). However, the association between LA substrate and outcome of catheter ablation (CA) for AF has not been fully elucidated. Methods and Results: A total of 31 patients with longstanding persistent AF underwent a stepwise ablation approach, which included complete isolation of the posterior LA including all pulmonary veins (PVs) with single continuous line, named endocardial-Box ablation, followed by electrogram-based ablation in both atria and then cavotricuspid isthmus block. A LA voltage map before CA was obtained with an electroanatomic mapping system (NavX) during AF and LA volume was calculated by axial slice summation from 3-dimensional computed tomography. After a stepwise ablation, conversion to sinus rhythm (SR) was in 24 patients (77.4%), directly from AF in 17% and through ATs in 83%. At a mean follow-up of 28짹8.7 weeks, 20 of patients (64.5%) were in SR without antiarrhythmic drugs after single procedure. Between patients with and without recurrence, there were no significant differences in age, sex, underlying diseases, body mass index, C-reactive protein, LA volume, left ventricular diastolic, and systolic functions. However, LA voltage was significantly lower in patients with recurrence compared to those without recurrence (0.69짹0.33 mV vs. 1.74짹1.01 mV, P=0.019). In multivariate analysis, LA voltage was only independent predictor for recurrence after single procedure (P=0.047). Conclusion: Left atrial voltage during AF was an independent predictor of single procedure outcome after a stepwise approach including endocardial-Box ablation in patients with longstanding persistent AF.