Coronary artery bifurcation lesion is still one of the most challenging lesion subsets in the field of non-surgical treatment for a stenotic coronary artery. When one stent is placed in the main brach, it increases the side branch's stenosis degree. However, its mechanism and incidence are not known. This study will be performed to search for the mechanism and incidence of that phenomenon.

Inclusion Criteria:

1. Elective coronary intervention (including stabilized ACS patients)
2. Bifurcation lesion with TIMI 3 flow
3. Side branch diameter > 2mm, side branch lesion length < 10mm by visual estimation

Exclusion Criteria:

1. Left main coronary disease
2. AMI, or Old MI at LAD territory
3. LVEF < 40%, or other significant valvular or myocardial disease
4. Significant side branch distal lesion
5. Angiographically visible thrombus
6. Heavily calcified lesion
7. Side branch predilatation before main branch stent implantation

Patients with bifurcating coronary artery lesion in left anterior descending coronary artery