Objective To observe the effect of accessory pathway (AP) conduction on PJ interval in patients with Wolff-Parkinson-White syndrome. Methods 129 patients with a single manifestation of AP who underwent successful radiofrequency ablation (RFCA) were included. Patients were divided into 10 groups according to AP location. The PR intervals, QRS durations and the PJ intervals were measured using simultaneous 12-lead ECG before and after ablation. The PJ intervals before ablation were compared with that after ablation. The atrioventricular (AV) conduction time via atrioventricular node-His conduction system before ablation were compared with the PR intervals after ablation. The ventricular depolarization time via atrioventricular node-His conduction system before ablation were compared with the QRS durations after ablation. Delta waves were compared between each two groups. Results (1) The PJ intervals of right posterior (RP) group and right posteroseptal (RPS) group before ablation were shorter than that after ablation (RP group 226±18 ms vs 236±19 ms, P<0.01, RPS group 221±18 ms vs 238±31 ms, P<0.05, respectively). (2) There were no significant differences between the atrioventricular (AV) conduction time via atrioventricular node-His conduction system before ablation and the PR intervals after ablation. (3)The ventricular depolarization time via atrioventricular node-His conduction system of RP group and RPS group before ablation were shorter than the PR intervals after ablation(RP group 79±12 ms vs 87±9 ms, P=0.01; RPS group 70±13 ms vs 86±9 ms, P<0.05, respectively). (4)The delta waves of RP group and RPS group were longer than that of left posterior group and left posteroseptal group(P<0.05). Conclusion PJ interval is shortened by AP conduction which pre-excites the general last excited part of left ventricle. It is determined by AP location and the extent of preexcitation. Objective To observe the effect of accessory pathway (AP) conduction on PJ interval in patients with Wolff-Parkinson-White syndrome. Methods 129 patients with a single manifestation of AP who underwent successful radiofrequency ablation (RFCA) were included. Patients were divided into 10 The PR intervals, QRS durations and the PJ intervals were measured using simultaneous 12-lead ECG before and after ablation. The PJ intervals before compared with that after ablation. The atrioventricular (AV) conduction time via atrioventricular node-His conduction system before ablation were compared with the PR intervals after ablation. The ventricular depolarization time via atrioventricular node-His conduction system before ablation were compared with the QRS durations after ablation. ) The PJ intervals of right posterior (RP) group and right posteroseptal (RPS) group before ablation were sh orter than that after ablation (RP group 226 ± 18 ms vs 236 ± 19 ms, P <0.01, RPS group 221 ± 18 ms vs 238 ± 31 ms, P <0.05, respectively). (2) There were no significant differences between the ventricular depolarization time via atrioventricular node-His conduction system of RP group and RPS group before ablation were shorter than the PR intervals after ablation (RP group 79 ± 12 ms vs 87 ± 9 ms, P = 0.01; RPS group 70 ± 13 ms vs 86 ± 9 ms, P <0.05, respectively). (4) The delta waves of RP group and RPS group were longer than that of left posterior group and left posteroseptal group (P <0.05). Conclusion PJ interval is shortened by AP conduction which pre-excites the general last excited part of left ventricle. It is determined by AP location and the extent of preexcitation.