We have studied a new type of myocardial catheter ablation with photosensitization reaction to realize non-thermal therapy for atrial arrhythmia, such as atrial fibrillation. Photochemically-generated reactive oxygen species may induce myocardial electrophysiological damage without heat generation. In this study, to demonstrate photosensitization reaction-induced myocardial electrical conduction block, the inferior vena cava to tricuspid annulus (IVC-TA) isthmus linear ablation was conducted with photosensitization reaction in porcine heart in vivo, using a newly developed laser catheter (7 Fr.). The end point of the procedure was the production of IVC-TA isthmus block under the electrophysiological analysis by diagnostic catheter with 10-bipole electrodes placed in right atrium along the isthmus. Talaporfin sodium (NPe6) as a photosensitizer was injected intravenously to pigs at 2.5-5.0 mg/kg. About 15 min after the injection, the laser light at the wavelength of 663 nm with a catheter output power density of 40-60 W/cm2 in about 1.4 mm spot size was irradiated through the laser catheter point by point in line crossing the isthmus under the fluoroscopic guidance. Before the photosensitization procedure, pacing signal from the distal electrodes of the diagnostic catheter, propagated through the isthmus in order. During the irradiation, electrical potential at the irradiated area was diminished. After the completion of the irradiation line, the bidirectional conduction block on the IVC-TA isthmus was validated by pacing from the distal and proximal bipole. These results indicated that photosensitization reaction could achieve the electrical conduction block of myocardial tissue immediately after the irradiation. We think that photosensitization reaction could become a novel therapy for atrial arrhythmia.