Background: Arrhythmias are one of the common presentations to critical units after ischemia or electrolytes imbalance, any rhythm apart from sinus rhythm is called arrhythmia. Timely interpretation of arrhythmia saves life mostly of the person who is the first responder to CCU, or ICU setup, which are the nursing staff. Aim of the study: The current study was conducted to assess factors affecting nurse's performance toward life-threatening arrhythmias for critically ill patient. Research Design: A descriptive design was used. .Study Settings: the study was conducted at cardiac intensive care units, General Intensive Care Units at Ain Shams University Hospitals. Subject: convenient sample of all available nurses working in pre mentioned settings and the study sample composed of 50 nurses. A purposive sample of 100 patients in pre mentioned setting. Result: (56%) of the studied nurses had total unsatisfactory knowledge about life-threatening arrhythmias. Also (76%) of them had total satisfactory practice. Additionally (58%) of the studied group had positive attitude towards the care of patients with life-threatening arrhythmias. Related to work environment (64%) of the studied nurses demonstrated negative regarding effect total factors related to work environment. Conclusion: More than half of the studied nurses had total unsatisfactory knowledge of life-threatening arrhythmias. Also more than three quarters of them had total satisfactory practice. Additionally more than half of them had positive attitude towards the care of patients with lifethreatening arrhythmias and less than two thirds of them demonstrated negative effect regarding total factors related to work environment. Recommendations: An orientation program should be prepared to help the newly appointment nurses to revise, acquire and develop their knowledge and practice regarding CPR, ECG. Specific courses about the recognition and management of Life-Threatening arrhythmias must be establish for critical care nurses to enhance them to be more knowledgeably, practice and attitude.