This report describes a case of permanent junctional reciprocating tachycardia (PJRT) that was ablated via the middle cardiac vein, guided by monophasic action potential recording. The patient was a 63-year-old woman who had been suffering from palpitation for 10 years. ECG during palpitation showed a narrow QRS tachycardia with a long RP interval. Electrophysiological study revealed that this tachycardia was an orthodromic reciprocating tachycardia, via an accessory pathway with a decremental property and a long ventriculoatrial interval (130 ms): PJRT. The earliest atrial activation during tachycardia was detected at the junction of the middle cardiac vein with the coronary sinus. Monophasic action potentials were recorded to confirm that the ablation catheter was in contact with the epicardium.