Purpose: This study was conducted to identify better methods of determining the severity of triage by comparing triage results and clinical outcome of patients categorized by the modified Canadian Triage Acuity Scale (mCTAS) and modified Emergency Severity Index (mESI). Methods: Subjects enrolled in this study consisted of 1,000 adult patients (age 16 years or older) who visited the emergency room of a university affiliated hospital between September 15, 2011 and September 30, 2011 and were categorized into five levels by mCTAS and mESI. Results: 1) Good confidence was verified based on weighted kappa values of 0.70 between the physicians group and nurses group. 2) Upon evaluation of triage by mESI, the majority of patients were at level 3 among 5, followed by level 4, 2, 1 and 5 in order. The same level orders were shown upon evaluation of triage by mCTAS beside differences in patient numbers. 3) Comparing clinical outcome according to the mCTAS and the mESI revealed similar results in both triage tools, with a higher triage level being associated with a higher admission rate and lower triage level and the discharge rate became higher. Conclusion: Triage by mESI showed good agreement among asserters and high agreement between physicians and nurses. Clinical results based on mCTAS and mESI triage showed similar rates of admission to the ward or intensive care unit and rates of discharge. Although these two triage protocols are similar in many aspects, the use of mESI is perceived as a better because mCTAS requires knowledge of various diseases and mESI has a short training period.