Background: A hospital's emergency department) ED) is the first contact point in the healthcare system. Triage system is the vital structure by which all incoming emergency patients are prioritized using a standard triage rating scale based on urgency. Aim of study: The study aimed to assess nurses' performance regarding triage in emergency unit. Design: A descriptive exploratory research design was utilized to achieve the aim of this study. Setting: The study was conducted in three emergency units at Ain Shams University Hospital; Surgical emergency unit, medical emergency unit and new emergency care unit. Study subjects: A convenient sample of 100 nurses who represent all available nurses in the previous mentioned setting. Data collection tools: I- Nurses' Knowledge Assessment Questionnaire which included; demographic characteristics, hospital emergency unit data and nurses' knowledge II- Nurses' Triage Competencies Observational Checklists. III- Nurses' Attitude Measuring Scale. Results: The study showed that, the mean age of the studied nurses was 30.98 ± 8.22 years, 77.0% of nurses were female, 46.0% of them graduated from technical nursing institute and 53.0% of them had 5-<10 years of experience. Regarding level of knowledge and practice, 56.0% of the studied nurses had unsatisfactory level of knowledge, 51.0% of them had incompetent level of practice and 54.0% of the studied nurses had negative attitude related to patients' triage in the emergency unit. Conclusion: The study findings concluded that, more than half of studied nurses had unsatisfactory level of knowledge, incompetent level of practice and negative attitude related to patients' triage in the emergency unit. Also, there was a significant positive correlation between total level of knowledge and level of practice and attitude. Recommendation: On-going and regular in –services educational programs regarding evidence –based guidelines should include the nurse performances regarding triage in emergency unit.