Background: Whether low molecular weight heparin (LMWH) has a superior efficacy to unfractionated heparin (UFH) in patients (pts) with acute Non-ST segment elevation myocardial infarction (NSTEMI) undergoing primary percutaneous coronary intervention (PCI) is still remains controversial. Method: A total 841 acute NSTEMI pts in KAMIR were enrolled. All the pts were assigned to receive either LMWH (60mg IV or SC bid for 5-7 days) or UFH (5,000 U IV bolus, followed by 24,000 U/day for 2days) as the antithrombotic regimen. The clinical outcomes of in-hospital, 1 and 6 months were compared. Result: Baseline clinical characteristics were similar between the two groups. These two groups also showed similar clinical efficacy profiles without significant difference in mortality, revascularization and all MACE up to 6 months. Furthermore, LMWH and UFH group also had similar in-hospital safety profiles without difference in major bleeding events (see Table). Conclusion: LMWH and UHF had similar safety and efficacy profiles with low incidences of MACEs and bleeding events in pts with acute NSTEMI undergoing primary PCI.