 ACS patients undergoing DES implantation who receive planned 600mg loading dose clopidogrel pretreatment are eligible for the study. All enrolled patients will be randomized to receive daily clopidogrel 75 mg or 150mg as maintaining doses starting as soon as post-PCI, in addition to daily aspirin 100 mg, and lasted for the first month after DES implantation. One month later, all patients receive daily clopidogrel 75mg until 912month after DES implantation. The primary endpoints include death of all causes, myocardial infarction, revascularization of the target lesson, sub-acute and late stent thrombosis one year after PCI, The secondary endpoints are major and minor bleeding events. The study will be powered to test the hypothesis that higher maintaining dose150mg of clopidogrel will reduce major adverse cardiac events compared to currently used common dose75 mg at one year following PCI. 