Introduction: Intracardiac defibrillation (IDF) is performed to restore sinus rhythm (SR) during radiofrequency catheter ablation (RFCA) of atrial fibrillation (AF). This study aimed to examine the change in the IDF threshold before and after RFCA during the procedure, and furthermore, to evaluate whether the IDF threshold after RFCA was associated with the AF substrate and recurrence of AF. Method: This study enrolled 141 consecutive patients with drug-refractory persistent AF (age 62.5±10.3 years, 84.4% male). Before the RFCA, we initially performed IDF with an output of 1 J. When the defibrillation failed to restore SR, the output was gradually increased up to 30 J. After RFCA, we attempted pacing-induced AF to provoke other focuses of AF. When AF was induced, we again performed IDF to terminate the AF with outputs of 1 to 30 J. The change in the IDF threshold to restore SR before and after RFCA was assessed. Results: The IDF threshold for restoring SR significantly decreased after RFCA (from 11.5±8.6 J to 4.0±3.8 J, p < 0.001). During the follow-up (24.3±12.2 months), SR was maintained in 107 patients (75.9%). A multivariate analysis using a Cox proportional hazards model revealed that an IDF threshold of > 5 J after the RFCA was significantly associated with recurrence of AF (HR, 3.99; 95% CI 1.93-8.22; p=0.0001). Conclusion: RFCA decreased the IDF threshold for restoring SR in patients with persistent AF. IDF outputs of > 5 J after RFCA could be a predictor of AF recurrence independent of the AF substrate.