Backgrounds: The predictor of functionally significant side branch (SB) stenosis after main branch (MB) stent implantation in bifurcation lesions is not well-known. We sought to identify the predictors of functionally significant SB jailing using anatomical and physiologic evaluations. Methods: Seventy seven patients with de novo bifurcation lesions who underwent provisional SB intervention were prospectively enrolled from 8 Korea, Japan and US centers. MB IVUS was performed before MB stent implantation and fractional flow reserve (FFR) was measured in the jailed SB. Results: The mean FFR was 0.71짹0.20 and 29 lesions (43%) were functionally significant. FFR was <0.75 in 18 out of 33 lesions (54.5%) with >75% diameter stenosis. Among pre-invervention angiographic and IVUS parameters, SB minimal lumen diameter (r=0.48, p<0.0001), SB percent stenosis (r=-0.47, p<0.0001), SB minimal lumen area (r=0.47, p=0.01), lumen volume index of distal MB (r=-0.35, p=0.02) and plaque volume index of proximal MB (r=-0.30, p=0.05) correlated with SB FFR. Binary logistic regression analysis revealed that pre-intervention % diameter stenosis of SB (OR= 1.05, 95% CI: 1.01-1.09) and MB minimum lumen diameter located at the distal to SB ostium (OR =3.86, 95% CI: 1.03-14.43) were predictors of functionally significant SB jailing (FFR<0.75). In patients with >75% stenosis and TIMI 3 flow in the SB, no difference in angiographic or IVUS parameters was found between SB lesions with and without functional significance. Conclusions: Anatomic evaluation does not reliably predict the functional significance of a jailed SB stenosis.