Background: The under-expansions after coronary stenting, especially in patients with acute coronary syndrome (ACS), are associated with post stenting cardiac events such as stent thrombosis. However, impacts of plaque components on that, affected by multiple factors, are not well established. Methods: We assessed 124 ACS (UA 37, NSTEMI 48, STEMI 39) pts treated with drug eluting stent (DES). Virtual histology intravascular ultrasound (VH-IVUS) imaging was performed before and after DES implantation. We measured a minimal stent area (MSA) and defined an under-expansion as less than 5 mm2 of MSA after stent deployment. We compared the plaque components of MSA site between well-expanded stent group (WEG) and under-expanded stent group (UEG) by VH-IVUS. Results: Pt age was 61짹12yrs and 86% were males in WEG; pt age was 61짹11yrs and 71% were males in UEG. Grayscale IVUS showed that WEG had larger lumen area (p=0.001), vessel area (p<0.001), and plaque burden (p=0.005) than UEG (Table). Remodeling index was not different (0.93짹0.21 in WEG vs 0.94짹0.23 in UEG, p=0.885). After DES implantation VH-IVUS showed that the dense calcium (DC) (p=0.021) and necrotic core (NC) (p=0.029) components were significant higher, while the fibrous component (p=0.023) significant lower in UEG compared to WEG (Table). Conclusion: The NC and DC components may play a role in under-expansion of coronary stenting in patients with ACS treated with DES as much as vessel size or amount of plaque burden.