Background: Several studies have suggested that pre-treatment of high clopidogrel loading dose (LD) increased more inhibition of ADP induced aggregation during periprocedural periods and improved in-hospital outcome in patients undergoing percutaneous coronary intervention (PCI). Our study aims to evaluate the efficacy of high clopidogrel LD in patients with ST elevation myocardial infarction (STEMI) underwent primary PCI. Methods and Results: A total of 545 patients, who diagnosed STEMI underwent primary PCI at CNUH between July 2006 to June 2008, were divided into pre-treated high clopidogrel LD (more than 600 mg loading, group I, n=256) and conventional clopidogrel LD (300 mg loading, group II, n=289). We compared in-hospital mortality and 1-month major adverse cardiac and cerebrovascular events. Door to balloon times were shorter in group I (445.7짹1031.2 vs 717.4짹1417.6, p=0.01), however, pre-PCI TIMI antegrade flow 0 or 1 and post-PCI TIMI antegrade flow 3 were no differences between two groups (58.6% vs. 59.2%, p=0.89 and 96.9% vs. 98.6%, p=0.17). Group II tended to receive triple antiplatelet thereapy after PCI (62.8% vs. 70.5%, p=0.07). TIMI major or minor bleeding events did not occur during periprocedural periods in both two groups. There were no differences in the occurrence rate of no-reflow phenomenon during procedure and the incidence of stent thrombosis (definite or probable) within 1 month between two groups (7.0% vs. 5.9%, p=0.59 and 2.1% vs. 0.7%, p=0.18). Also, there were no difference in in-hospital mortality and 1-month MACCE (5.5% vs. 4.2%, p=0.47 and 2.6% vs. 1.9%, p=0.58). In multivariate analysis, female gender, creatinine clearance, pre-PCI TIMI antegrade flow 0, ejection fraction, and left main disease were the independent predictors of in-hospital mortality (HR=3.984; 95% CI=0.005-0.954, HR=6.404; 95% CI=0.847-0.979, HR=4.261; 95% CI=1.139-151.062, HR=6.592; 95% CI=0.785-0.968, HR=5.704; 95% CI=1.990-1078.255) in patients with STEMI. Conclusion: Our data show that high clopidogrel loading dose strategy is not superior to conventional loading dose strategy on early outcomes in patients with STEMI underwent primary PCI.