It has been reported that coronary vasa vasorum is associated with plaque vulnerability, and low-echoic structures in grayscale intravascular ultrasound (IVUS) are consistent pathologically with vasa vasorum. However, the association of low-echoic structures with plaque composition and no-reflow phenomenon during percutaneous coronary intervention (PCI) is unclear. We investigated plaque composition in virtual histology IVUS (VH-IVUS) and no-reflow phenomenon during PCI of low-echoic structures.A total of 106 lesions being treated by VH-IVUS before PCI were included in this study. Low-echoic structure was defined as a small tubular structure exterior to media without a connection to the vessel lumen in ≥ 3 consecutive crosssectional IVUS images. Lesions with low-echoic structures were found in 42% (45/106).Lesions with low-echoic structures were more prevalent in acute coronary syndrome (ACS) patients (53% [24/45] versus 20% [12/61], P < 0.001), had more positive remodeling (49% [22/45] versus 21% [13/61], P = 0.003), a larger number of VH-IVUS derived thin-cap fibroatheromas (VH-TCFAs) (0.64 ± 0.53 versus 0.05 ± 0.22, P < 0.001), more VH-TCFAs with a baseline plaque burden of 70% or more and minimal luminal area of 4.0 mm(2) or less (29% [13/45] versus 2% [1/61], P < 0.001), and more frequent no-reflow phenomenon after stent implantation and more final TIMI flow grade 0/1/2 (38% [17/45] versus 5% [3/61], P < 0.001; 9% [4/45] versus 0% [0/61], P = 0.03) than lesions without low-echo structures.Lesions with low-echoic structures in grayscale IVUS had high plaque vulnerability and were more prevalent in ACS patients, positive remolding, and VH-TCFAs, and they had more frequent no-reflow phenomenon during PCI than lesions without low-echoic structures.