Purpose: The aim of this study was to measure the inter-rater reliability of Emergency severity index (ESI) version 4 among triage nurse. Methods: This study was carried out from August 11, 2010 to September 7, 2010 in a regional emergency department. Data collection was done by ten triage nurses who trained ESI v.4. Two research nurses and ten triage nurses scored the ESI version 4 to the patients as references, independently. We calculated the weighted kappa between the triage nurses and research nurses to evaluate the consistency of the ESI v.4. Results: A total of 233 patients were enrolled in this study. Classification of ESI level was as follows - level 1 (0.4%), level 2 (21.0%), level 3 (67.8%), level 4 (9.4%), and level 5 (1.3%). Inter-rater reliability by weighted kappa was 0.79 (95% Confidence Interval= 0.74-0.83) and agreement rate was 87.1%. Under-triage rate by triage nurse was 6.0% and over-triage rate was 6.9%. Conclusion: For this study, inter-rater reliability was measured good level between triage nurses and research nurses in Korean single ED.