Background: The pediatric nurses in emergency care settings are the first persons whom the patient encounters, and the nurses' knowledge of the triage system has been cited as an influencing factor in the triage decision. Triage skills are core competency required for emergency nurses. This study aimed to evaluate the effectiveness of teaching guidelines regarding pediatric triage assessment and management of critically ill children on nurses' performance. Research design: This study used a quasi-experimental research design. This study consisted of thirty-eight nurses working at Pediatric Intensive and Emergency Care Units in Sohag University and General Hospital. A pre-designed interview questionnaire (pre-posttest) was used to collect the data: It consisted of 3 parts; part 1 about nurses' personal characteristics, part 2 to assess nurses' knowledge regarding pediatric triage system and part 3 about assessment and management of critically ill children's guidelines according to WHO, (2013). Results: The nurses' overall mean of knowledge in the pre-test was 8.05±2.25 and their practice was 20.89±1.66, while, their mean knowledge and practice was significantly improved (p = 0.000), where they were 25.53±2.91 and 27.82±3.68 immediately after implementation of the guidelines. Conclusion: Teaching guidelines had highly statistically significant positive effect on improving the studied nurses' performance regarding pediatric triage assessment and management of critically ill children. In addition, there was statistically significant moderate positive correlation between triage knowledge and triage practice among the studied nurses. Recommendation: Implementing the triage system in all pediatric emergency care to obtain appropriate and effective priority for patients, which in turn will reduce the financial burdens and overcrowding the problems in emergency settings.