Introduction; Atrial fibrillation(AF) in patients with Wolf-Parkinson-White(WPW) syndrome is relatively frequent. Several mechanisms have been proposed to explain for this high incidence of paroxysmal AF in the WPW syndrome. However, the reason have not yet been fully explained. The purpose of this study was to assess differences in the clinical and electrophysiological characteristics in WPW patients with AF and without AF. Methods; This study was a retrospective study of patients with WPW syndrome referred for accessory pathway(AP) ablation due to clinical documented AVRT and/or AF, or atrial flutter or unknown cause of palpitation from 1998 to 2011 at Yonsei university severance hospital, Inha university hospital and Cha university hospital. Standard electrophysiological study(EPS) were performed to induce AVRT or AF. We compared clinical feature and electrophysiological feature of AP and AVRT to have an effect on AF occurrence between the patients with AF (group1), and those without AF (group2). Results; EPS was performed among 797(528 male) patients with WPW syndrome. The patients with AVRT, AVRT/AF, and AF were 611(76.7%), 77(9.7%), and 93(11.7%), respectively. Sixteen patients(2.0%) was not induced any tachycardia. Group1 were older than group2(42짹14 vs 37짹16). The frequency of multiple accessory pathway was similar between both group. Effective refractory period(ERP) of atrium were significant longer in group1 than group2. Anterograde ERP of AP was significantly shorter in group1 than group2. Significant, positive correlations between A-V and V-V were present in both groups for AVRT. But, the investigation of the correlation of the V-A to V-V revealed differences between both groups. V-A correlated significantly with the V-V time in group 2(r=0.104, P=0.030), whereas the absence of such correlation was observed in group 1(r=0.088, P=0.487). Conclusion: The appearance of AF in WPW syndrome was associated with older age, male gender, shortened ERP of AP, and maladaptation of retrograde conduction of AP for AVRT.