Electrophysiological study should be done in patients who suppose to undergo the treatment including pulmonary vein isolation using catheter ablation for atrial fibrillation. The purpose of the electrophysiological study consist with confirmation of the induction and the origin of atrial fibrillation, and with recognition of electrical potential and arrhythmogenic substrate for maintenance of atrial fibrillation. The shortening and irregularity of the refractory periods and prolongation of conduction time are important factor in occurrence of atrial fibrillation. Moreover, the complex fractionated atrial electrograms(CFAE) and ganglionated plexus(GP) play an important role in maintenance of atrial fibrillation. As it is indispensable to record and distinguish the electrical potential of atrium and pulmonary vein for the ablation of atrial fibrillation, every physician who is concerned in the catheter ablation requires accustoming to these potentials.