Background: There have been numerous parameters for differentiating atroventricular nodal reentrant tachycardia (AVNRT) from orthodromic reciprocating tachycardia (ORT) using a concealed bypass tract. The aim of this study was to evaluate the discriminating value of these parameters. Methods: This study included 106 consecutive patients who underwent radiofrequency catheter ablation for paroxysmal supraventricular tachycardia without pre-excitation at two medical centers. The right ventriclular apex was paced at a cycle length 20 to 40 ms shorter than the tachycardia cycle length (TCL). The ventriculo-atrial (VA) interval and TCL were measured just before pacing. The first post-pacing interval (PPI) and stimulus-atrial (SA) interval were also measured on cessation of ventricular pacing. Results: Between 69 patients with AVNRT and 37 patients with ORT, there were significant differences in mean values for VA interval, SA interval, SA-VA difference, PPI-TCL difference, and corrected PPI-TCL difference. Receiver operating characteristic (ROC) curves showed that the area under ROC curve for SA-VA difference was largest in differentiating AVNRT from ORT (0.817, p<0.001). The cutoff value of 85 msec for VA interval provided best discriminative value with a sensitivity of 81.2% and specificity of 62.2% in differentiating AVNRT from ORT. Conclusion: The SA-VA difference could be more useful parameters for differentiating AVNRT from ORT.