Background: The appropriate criteria for revascularization of intermediate coronary lesions have been under debate. Fraction flow reserve (FFR) is considered the gold standard for assessing such lesions. Intravascular ultrasound (IVUS) has become the more accurate standard for defining the anatomy of atherosclerosis in vivo. To date, available data regarding the relationship between anatomic IVUS parameters and functional FFR results have been from retrospective data analyses and are variable. This study aimed to determine the optimal minimum lumen area (MLA) by IVUS that correlates with FFR and to assess the correlation between two modalities in assessing intermediate coronary lesions. Methods: Fifty eight intermediate coronary lesions mainly located in proximal and mid segments of large main coronary vessels with RVD (3-4mm) were analyzed using both IVUS and FFR to assess the significance of coronary stenting and to determine the optimal IVUS-MLA that correlates with FFR value < 0.8. Results: Overall, IVUS-MLA ranged from 2.5 to 4.2 mm2 had a highly significant positive correlation with FFR value < 0.8 (p < 0.0001). Using the ROC curve analysis, IVUS-MLA < 3.9 mm2 (84.2% sensitivity, 80% specificity, area under curve (AUC) = 0.68) was the best threshold value for identifying FFR