 If TIMI flow grade 3, the subject will receive stent implantation as the standard of care followed by OCT examination; If TIMI flow grade of 3, the subject will not receive stent implantation but will be sent to CCU for monitoring and antiplatelet treatment, and have a repeat angiography and OCT examination at Day 7; within the 7days post STEMI, ischemia-driven angiography andor PCI are allowed, and an OCT examination will be performed in these subject before PCI. 