Introduction: Transesophageal cardiac electrophysiological study can identify the type of supraventricular tachycardia and may find some special electrophysiological phenomena. Case presentation: This paper reported a 1-month-old infant who underwent transesophageal cardiac electrophysiological study and was induced atrioventricular nodal reentrant tachycardia and atrial flutter. The type of narrow QRS tachycardia in the infant was defined, which showed great value for the selection of medications to terminate tachycardia in the near future and the long-term radiofrequency ablation therapy. Conclusion: Transesophageal cardiac electrophysiological study is viable，safe and non-invasive, additionally can help to find some important ECG phenomena. It is especially suitable for infants who are temporarily inaccessible to receive radiofrequency ablation.

Introduction: It is very rare that wide QRS complex tachycardia automatically changes to narrow QRS complex tachycardia, and it is more difficult to clarify the mechanism. Case report：A 3-month-old infant with recurrent paroxysmal supraventricular tachycardia underwent transesophageal cardiac electrophysiological examination. The wide QRS complex tachycardia was induced by atrial RS2 stimulation, and it soon changed into narrow QRS tachycardia automatically. By the accurate measurement of esophageal lead, it was found that the electrocardiogram changes completely conform to Coumel law. Conclusion: Transesophageal cardiac electrophysiological examination can reveal certain special electrophysiological phenomena, especially suitable for infants.