Background: Some reports suggest that there might be a trend towards poorer outcome of ST Elevation Myocardial Infarction (STEMI) pateints treated by Primary Percutaneous Coronary Intervention (PPCI) during night hours. Methods: A total of 5912 patients with STEMI treated by PPCI were enrolled in KAMIR between October 2005 and December 2007. Patients were divided into two groups according to thier time of arrival to emergency room, from 9 am - 5:59 pm (Day-shift) and from 6 pm - 8:59 am (Night-shift). Results: There were 2610 patients admitted during Day-shift and 3302 patients during Night-shift. Patients in both groups did not differ in baseline demographics and clinical status except age. Age of the patients during night-shift was younger than that during day-shift (60.9짹13.5 vs 62.2짹13.3, p=0.0001). Overall time from door to balloon time were 171.4짹43 min in Day-shift and 201.6짹232 min in Night-shift respectively (p=0.129). Infarct related artery patency (TIMI 2 and 3 flow) in baseline angiography (26.2% vs 27.3%, p=0.395) and post procedure (86.7%, vs 88.0%, p=0.515) were similar respectively. There were no difference in in-hospital mortality, 1 month MACE and 12 months MACE. Conclusion: The clinical outcomes of STEMI patients treated by PPCI during Night-shift are not inferior to those during day-shift.