Abstract: Unsynchronized atrioventricualr activation during atrioventricular nodal reentranttachycardia (AVNRT) were seen in 13 patients, 11 of which had 2: 1 AV conduction, 2 AV dissocua-tion. This phenomenon illustrates that the atrium and the ventricle were not the necessary part of the reen-trant circuit. Whether the atrium can be 1: 1 involved in tachycardia depends on the effective refractoryperied (ERP) of Perinodal atrial tissue and the ventrivular depends on that of His - purkinje system. AVblock AV dissociation is common and VA block or VA dissociation uncommon because of longer ERP of His- Purkinje system and shorter ERP of atrial tissue. The recognization of the unsynchronized atrioventricu-lar activation phenonmenon during AVNRT is very important in diagnosing and radiofrequency catheter ab-lation of AVNRT.