Background: Although radiofrequency catheter ablation (RFCA) of catheter ablation is an effective rhythm control strategy in patients with atrial fibrillation (AF), its mechanism remains to be clarified. Defibrillation threshold (DFT) and inducibility are parameters reflecting critical mass. We hypothesized that RFCA of AF would reduce atrial DFT (ADFT) and inducibility by critical mass reduction. Method: We evaluated ADFT and AF inducibility in 39 patients with AF (male 84.6%, 52.3짹11.7 years old. 24 paroxysmal AF [PAF]: 15 persistent AF [PeAF]) who underwent RFCA before procedure and after bi-antral ablation of 4 pulmonary veins with/without additional linear ablations. ADFT was determined by serial internal cardioversion (biphasic shock 2J, 3J, 5J, 7J, and then 10J from high right atrium [HRA] to coronary sinus [CS]). AF inducibility was tested by rapid HRA or CS pacing with 150ms 1:1 capture. RFCA was performed by open irrigation tip catheter and electrical PV isolation or bi-directional block along the linear ablation were confirmed. Results: 1. RFCA of AF reduced ADFT from 6.0짹3.6 J before procedure to 3.8짹2.8 J after procedure (p < 0.01). 2. AF ablation reduced AF inducibility from 84.6% before procedure to 38.5% after procedure (p < 0.01) and induction pacing cycle length was shortened from 192.3짹40.4 ms to 157.1짹30.0 ms (p < 001). 3. ADFT reduction effects after RFCA were not significantly different between PAF and PeAF (4.8짹3.5 J to 2.7짹1.2 J in PAF vs. 7.9짹3.0 J to 5.0짹3.5 J in PeAF ; p = 0.78) but inducibility reduction effects was significantly different between two groups (79.2% to 20.8% in PAF vs. 93.3% to 66.7% in PeAF ; p = 0.007). Conclusion: RFCA of AF reduces ADFT and AF inducibility significantly, and reduction of atrial critical mass is one of the important anti-arrhythmic mechanisms. Therefore, appropriate ablation line and completeness of conduction block might be essential for the better clinical outcome after AF ablation.