Objective: To evaluate the validity, reliability, sensitivity, and specificity of the Emergency Severity Index (ESI) and Australasian Triage System (ATS) for children visiting admitted to the emergency department (ED).Methods: This was a prospective study occurred in the Mofid children's Hospital in Iran from August 2017 to November 2018 and children had aged ≤14 years and presented at the ED with a medical symptom were considered eligible for participation. This study was divided into two phases: in the first phase, we determined the inter-rater reliability of ESI version 4 and ATS by triage nurses and pediatric residents. In the second phase, to analyze the validity, sensitivity, and specificity of each triage system. Reliability and agreement rates were measured using kappa statistics.Results: ESI showed inter-rater reliability with kappa of 0.65–0.92 (P<0.001) and ATS showed inter-rater reliability with kappa of 0.51–0.87 ESI had sensitivity ranged from 81% to 95% and specificity ranged from 73% to 86%. In addition, sensitivity ranged of the ATS were 80% to 95% and specificity ranged from 74% to 87%. Under triage and over triage occurred in 12% and 15% of patients respectively in ESI and 13% and 15% of patients respectively in ATS.Conclusion: The ESI and ATS both valid to triage children in the ED section of Mofid children's Hospital paediatric. Reliability of the ESI is good, moderate to good for the ATS.