Objectives and Background; Stent thrombosis(ST) is a serious complication of drug-eluting stent (DES) implantation regardless of the timing (acute, subacute, or late). FDA panel accepts stent thrombosis but rejects increased death/MI risk for on-label DES use. To date, the safety and efficacy of Intravascular Ultrasound(IVUS) use in DES implantation has been well evaluated but few data of IVUS for ST. We evaluated the impact of IVUS for ST in DES implantation. Method: A total 1,448 patients with DES implantation were included from Jan 2006 to Feb 2008. We divided phase I (from Jan 2006 to Dec 2006, before IVUS use, 731 patients) and phase II (from Mar 2007 to Feb 2008, after IVUS use, 717 patients) ST was defined by Academic Research Consortium (ARC) as reported in TCT 2006 meeting. Result; The median follow-up duration was 6 months in both phase. 31 (2.1%) stent thrombosis occurred: 21 ST (2.9 %) in phase I (4 subacute, 17 late ST) and 10 (1.4%) ST (4 subacute, 6 late ST). The incidences of stent thrombosis in phase I was higher than phase II (2.9% vs 1.4 %, p=0.052) but not significant. Subacute ST in phase I wasnot higher than phase II (0.5% vs 0.6 %, p=0.978). Late ST in phase I was significantly higher than phase II (2.3% vs 0.8 %, p=0.023). The univariate logistic regression shows the hazard ratio for late ST was 0.376 (95% CI; 0.146-0.971) for IVUS use and 0.959 (95% CI; 0.936-0.981) for HDL. After adjustment for age, sex and HDL, IVUS use showed the trend for predicting late ST (hazard ratio: 0.223 , 95% CI; 0.049-1.003, p=0.050). Conclusion :The incidence of late stent thrombosis was higher in the phase I before IVUS use compared with the phase II after IVUS use. The use of IVUS showed the trend for decresing late ST but the large scale study was needed for the evaluation of the impact of IVUS use in DES implantation.