Background & Objectives: It is well known that ostial lesion is difficult to evaluate stenosis severity by coronary angiography. Fractional flow reserve (FFR) is very specific physiologic parameter which reflect the degree of stenosis. This study was performed to compare the anatomic stenosis severity assessed by coronary angiography with FFR in ostial lesion. Methods: We enrolled seventy-five patients with angina who has ostial lesion with intermediate stenosis (30-70%) by coronary angiography. We measured FFR with continous intracoronary adenosine infusion via microcatheter and performed intravascular ultrasound in fifty-seven patients. Results: Only 33% of patients with angiographic percent diameter stenosis (%DS) > 50% were FFR < 0.80 and 22% of patients with %DS > 50% were FFR < 0.75. The best cutoff values for functional significance to discriminate a FFR < 0.8 versus > or = 0.8 were angiographic percent diameter stenosis (%DS) of > 57 % (Area under curve(AUC) 0.689, 95% confidence interval (CI) 0.544-0.833), minimal luminal diameter (MLD) of < or =1.5 mm (AUC 0.709, 95% CI 0.568-0.849) and minimal lumen area (MLA) of < or = 3.9 mm짼(AUC 0.711, 95% CI 0.554-0.867). The best cutoff values for functional significance to discriminate a FFR < 0.75 versus > or = 0.75 were angiographic %DS of > 63 % (AUC 0.812, 95% CI 0.644-0.981), MLD of < or =1.32 mm (AUC 0.786, 95% CI 0.615-0.956) and MLA of < or = 3.55 mm짼 (AUC 0.712, 95% CI 0.559-0.865). Conclusions: In ostial lesions, anatomic severity of a coronary stenosis measured by quantitative coronary angiography appears to be somewhat overestimated compare with FFR. FFR measurement for ostial lesions are feasible and helpful to evaluation of functional significance and clinical decision making.