Emergency Unit is also called Emergency Room ( ER), the department of hospital responsible for the provision of medical and surgical care to patients arriving at the hospital in need of immediate care. Emergency personnel may also respond to certain situations within the hospital such cardiac arrests. Nurses' knowledge and practice play important roles in provision of supportive care to decrease morbidity and mortality of patient who arrive Emergency and need immediate care. Aim of the study to assess Factors affecting nurses' performance in Emergency unit. Study design: a descriptive exploratory design. Subject: Convenience sample of 40 nurses were recruited from Emergency unit Unit at Ain Shams University Hospital within 6 months duration. Tools: consisted of (1) Self-administered questionnaire sheet to assess nurses' knowledge regarding Emergency unit, ,and Self-administered questionnaire sheet to assess factors affecting nurses' performance in Emergency unit (2) Nurses' practice observational checklists to assess nurses' practice regarding most common procedure in Emergency unit. The Results of this study revealed that; 55 % of the nurses were under the age of 40 years, 60.5 % of nurses had unsatisfactory level of knowledge, 90.0 % of them had inadequate level of practice, 100.0 % of them agreed on personnel factors negatively affecting their performance in Emergency unit . Recommendation: this study recommended; Continuous evaluation of nurses' knowledge and practice is essential to identify nurses' needs and factors affecting their performance in Emergency unit. Close supervision and teaching on spot during work is needed to ensure that quality of care is provided by nurses in Emergency unit. Developing continuing educational programs including evidence based guidelines based on needs assessment for nurses to improve their performance and quality of care regarding work in Emergency unit.