Post-pacing interval minus tachycardia cycle length (PPI-TCL) during right ventricular (RV) entrainment can help in differentiation between atrioventricular reentrant tachycardia (AVRT) and AV nodal reentrant tachycardia (AVNRT), however it has some fallacies and limitations. RV pacing during SVT produces progressive QRS fusion before the QRS morphology becomes stable. This transition zone (TZ) may differentiate AVRT from AVNRT independent of entrainment success. We thought to test atrial preexcitation (AP) & Stimulus to atrial (S-A) interval fixation in relation to TZ in identifying the mechanism of SVT.We studied retrospectively and prospectively the effect of properly-timed RVP (within 40 ms) in 118 SVT patients on atrial timing in relation to TZ. A fixed S-A interval was defined as variation ≤ 5 ms during RVP & AP is the first change in atrial CL ≥ 10 ms. 9 patients were excluded due to cycle length oscillation>10 ms before the onset of RVP and 6 patients had atrial tachycardia and were excluded. In 103 patients, PPI-TCL and cPPI-TCL were significantly longer in AVNRT patients but postpacing response couldn’t be assessed in 11 patients showing tachycardia termination with RVP entrainments. And PPI-TCL was not diagnostic in 6 patients with ORT (>115 ms) but all of them had cPPI-TCL < 110 ms. AP occurred during TZ in most AVRT patients and after TZ in most AVNRT patients. However, S-A fixation occurred during TZ in all AVRT patients and after TZ in all AVNRT patients. Fixation of S-A interval and AP in relation to TZ were more sensitive than either PPI-TCL or cPPI-TCL in identifying the mechanism of SVT (100%, 91.5%, 83.5% and 89.5% consecutively).We can conclude that during RVP within 40 ms of the tachycardia cycle length, fixed S-A interval and AP in relation to FZ were superior to PPI-TCL and c PPI-TCL measurement in identifying the mechanism of SVT.