Objective To investigate the value of aVR/aVL QS-wave amplitude ratio on electrocardiogram in differentiation between posterior and anterior origins in right ventricular outflow tract (RVOT) ventricular arrhythmias (VA). Methods Clinical data of 87 VA patients who underwent successful radiofrequency catheter ablation with the target site at the RVOT during 2009 and 2019 in the First Medical Center of Chinese PLA General Hospital were collected and retrospectively analyzed. According to the target site for ablation which was determined by projection of 30° right anterior oblique X-ray, the patients were divided into anterior group (left half of the RVOT, n=52) and posterior group (right half of the RVOT, n=35). The QS-wave amplitudes in lead aVL and aVR during an episode of VA were calculated in both groups, and the relationship of their ratio, QSaVR/QSaVL, with the target site for ablation was analyzed. SPSS statistics 24.0 was used for data analysis. Receiver operating characteristic (ROC) curve was employed to calculate the sensitivity and specificity of QSaVR, QSaVL, and QSaVR/QSaVL in prediction of the anterior or posterior origin in RVOT-VAs, and the area under curve (AUC) was used to compare their accuracy. Results The QSaVR/QSaVL was significantly smaller for anterior origins than posterior origins in RVOT-VAs [(1.25±0.81) vs (2.13±1.16), P<0.001]. The AUC for the QSaVR/QSaVL was larger (0.786) than that for the QSaVL (0.724) or the QSaVR (0.649). With an optimal cut-off value of 1.48, a Youden index was the largest (0.54), and the sensitivity was 0.83, specificity 0.71 in prediction of an anterior origin. Conclusion The aVR/aVL QS ratio is of certain guiding significance in differentiation of anterior from posterior origins in RVOT-VAs.