Background: Optimal bare metal stent (BMS) implantation by intravascular ultrasound (IVUS) has been provided a lower angiographic restenosis as well as favorable clinical outcomes. It has been reported a significant degree of underexpansion still remained despite of nominal to high pressure balloon inflation. However, the importance of optimal stent implantation in the era of drug eluting stent (DES) is not well known. Method: This retrospective study included 126 patients (134 lesions) who underwent post-stent IVUS and follow-up coronary angiography. All patients were divided into two groups: DES group (n=61) and BMS group (n=65). A quantitative coronary angiographic (QCA) restenosis and major adverse cardiac events (MACE) were evaluated at 6 months follow-up. IVUS criteria for optimal stent expansion were defined as minimal stent area ratio was 0.8 of average reference lumen area or 0.9 of distal reference lumen area. Results: Optimal stent expansion by IVUS achieved 41% of patients in DES group and 54% in BMS group. A QCA restenosis rate and MACE were lower in DES group (p<0.01). Other data are in the following table. Conclusion: A significant degree of stent underexpansion was observed in both groups. Although vessel size and stent were smaller in DES group, a QCA restenosis and clinical outcomes were favorable than in BMS group. Therefore, the concept of optimal stent expansion in the era of drug eluting stent might be not as important as that in BMS.