Backgrounds: The objective of this study is to assess the relationship between intravascular ultrasound (IVUS) parameters and fractional flow reserve (FFR). Methods: Patients with intermediate coronary artery stenosis (30-70% diameter stenosis) were prospectively enrolled. Both IVUS and FFR were performed in 108 left anterior descending artery. Myocardial ischemia was assessed by FFR and maximal hyperemia was induced by intracoronary continuous adenosine infusion using microcatheter. We considered a value of the FFR < 0.80 as a significant inducible ischemia. We measured standard IVUS parameters (minimal lumen area, lesion length, plaque burden, area stenosis) and volumetric analyses (lumen, atheroma, vessel volume and percent atheroma volume (PAV)) were performed. We evaluated the relationship between FFR and IVUS parameters including volumetric analyses, also compared the IVUS parameters according to the significant ischemia was present (n=44) or not (n=64). Results: FFR was correlated with IVUS parameters including minimal lumen area (MLA) (r= 0.421, p< 0.001), plaque burden (r= -0.349, p< 0.001), lesion length (r= -0.369, p< 0.001) and percent area stenosis (r= -0.311, p= 0.001). FFR also correlated with total atheroma volume (r= -0.296, p= 0.002) and PAV (r= -0.327, p= 0.001) of target lesion. The best cutoff values of IVUS parameters to predict functional significance was 3.2 mm² (AUC 0.82, 95% CI 0.65-0.93) of proximal LAD and 2.5mm² of mid LAD for MLA (AUC 0.74, 95% CI 0.62-0.84), 21 mm for lesion length (AUC 0.82, 95% CI 0.73-0.89) and 70% for area stenosis (AUC 0.62, 95% CI 0.52-0.71), respectively. Volumetric IVUS analyses showed that the lesions with significant ischemia (FFR < 0.80) were larger total atheroma volume (204.1 짹 89.5 vs. 135.9 짹 89.4mm³, p< 0.001) and a PAV of target lesion (60 짹 6 vs. 56 짹 7%, p= 0.003). Conclusions: IVUS parameters which represent the severity and amount of atheromatous plaques were correlated with functional significance in intermediate coronary artery stenoses of LAD.