AIMS This study aimed to evaluate the electrocardiographic characteristics and predictors of successful ablation for ventricular arrhythmias (VAs) with superior axis originating in the ventricular septum. METHODS AND RESULTS This study included 385 consecutive patients with VAs undergoing radiofrequency ablation (RFA). Of these, 14 patients (3.7%) were identified who had VAs with superior axis that were mapped to and ablated at the left ventricular (LV) septum. These patients were classified into two groups, successful (n = 9, Success-RFA) and failed (n = 5, Fail-RFA) ablation. The QRS duration of the VAs was longer in the Success-RFA than the Fail-RFA [median (25%, 75% quartile), 140 (134, 149) vs. 128 (116, 132) ms; P = 0.007]. In the Success-RFA, the QRS morphology in lead V1 exhibited qR or rSR (r < 0.2 mV) pattern. In the Fail-RFA, QRS in lead V1 demonstrated an initial R-wave of ≥0.2 mV except for one patient who demonstrated a qR pattern. The initial R-wave amplitude of <0.2 mV in lead V1 identified successful ablation cases with 100% sensitivity and 80% specificity. The magnitude of the initial R-wave amplitude in lead V1 could be related to the originating site's depth within the septal tissue, which could also explain the RFA results. CONCLUSION Four percent of VA patients had superior axis on electrocardiogram and foci that mapped to the LV septum, two-thirds of which were successfully ablated. The initial R-wave amplitude of <0.2 mV in lead V1 identified RFA success with high sensitivity and specificity.