 In view of its safety profile and the results of clinical trials, clopidogrel has become the standard treatment for patients with acute coronary syndrome ACS and drug eluting stentDES implantation. Two large studies in patients with ACS shown that pretreatment with clopidogrel had beneficial effects. The pretreatment regimens were given a mean of 6 days before intervention in the observational PCI-CURE trial and 3 to 24 h in the randomized CREDO trial respectively. Accordingly, current clinical practice carries out pretreatment with a 300mg loading dose of clopidogrel at least 6 h before DES implantation procedure in patients with ACS. Compared with the 300mg clopidogrel loading dose, 600mg loading dose exhibited a superior antiplatelet effect and improved short-term clinical outcomes in patients undergoing DES implantation for ACS according to recent a few publications. But despite clopidogrel 600mg loading dose and the routine use of 75mg per day as a maintaining dose, recurrent ischemic events occurred in some patients. Therefore, the goal of this study will evaluate the efficacy of a 600mg loading dose of clopidogrel plus 150mg per day as a maintaining dose in patients with ACS undergoing DES implantation. 