The aim of this study was to investigate the incidence of binary restenosis and its predictors in patients with ostial lesions of the right coronary artery (RCAos) who underwent percutaneous coronary intervention (PCI). RCAos are associated with a high incidence of restenosis, and the implantations of drug-eluting stents for RCAos have not been fully elucidated. The study participants included 75 patients (72.3 ± 9.5 years, 72% men) who underwent PCI for RCAos at our institution between November 2001 and May 2017. The angle between the greater curvature of the aortic wall and the right coronary artery take-off in the diastolic and systolic phases in the left anterior oblique position view was investigated. Clinical outcome was defined as binary restenosis at follow-up coronary angiography. We also evaluated target lesion failure (TLF) defined as a composite of cardiac mortality, target vessel myocardial infarction, and target lesion revascularization (TLR). The incidence of binary restenosis was 48.0% (n = 36) of the entire cohort. The incidence of TLF was 49.3% (n = 37) of the entire cohort, which was mainly driven by TLR (36.0%, n = 27). The area under the curve of the gap-angle ratio [(difference between the maximum and minimum angles)/(minimum angle); GAR] for binary restenosis was 0.73, and the cutoff value was 0.306 (sensitivity 67%, specificity 82%). The patients were divided into two groups: a low-GAR (< 0.306; n = 30) and high-GAR group (> 0.306; n = 45). Binary restenosis was more frequent in the high-GAR group than in the low-GAR group (76.7% vs. 28.9%, p = 0.007). The cumulative rate of TLF was significantly higher in the high-GAR group when compared with the low-GAR group (53.3% vs. 40.0%, p = 0.01), which was mainly driven by TLR (56.7% vs. 22.2%, p = 0.01). High-GAR (> 0.306) [OR 2.66 (1.34–5.31), p = 0.005] and stent under expansion [OR 2.37 (1.10–5.11), p = 0.03] were found to be independent predictors of binary restenosis after adjustment for multiple confounders. Multivariable analysis also revealed that high-GAR (> 0.306) [OR 2.06 (1.02–4.14), p = 0.03] and stent under expansion [OR 2.82 (1.28–6.19), p = 0.01] were independent predictors of TLF. We suggest that GAR (> 0.306) predicts binary restenosis and TLF in patients undergoing PCI for RCAos.