Background: Arythmisas are a typical presentation to critical care units following ischemia or electrolytes imbalance ; an arrhythmia is any rhythm other than sinus rhythm. Timely interpretation of arrhythmia saves lives, particularly those of the nursing staff who are the first responders to medicine administrations in a CCU or ICU setting. Aim of the study: This study was conducted to assess nurses‟ level of knowledge regarding the administration of intravenous magnesium for patients with cardiac arrhythmia, Design: A descriptive exploratory design was utilized to achieve the aim. Setting: This study conducted at critical care units of two hospitals Cardiovascular and thoracic academy hospital and National Cardiac Institute Hospital. Affiliated with Ain Shams University Hospital. Sample: A convenient sample (50) of all available nurses were be included from previously mentioned setting. Tools : Data was collected using the following two tools 1) nurse self-administration questionnaire, Nurses‟2) Nurses‟ observational checklist Results: 86% majority of the studied nurses‟ have an unsatisfactory level of knowledge while only 14% of them had satisfactory level, regarding the total nurses‟ levels of total practical knowledge about guidelines on the safe use of intravenous magnesium. 90% of the studied nurses‟ had an unsatisfactory regarding total level of practice. Conclusion : the majority of nurses had an unsatisfactory level regarding of knowledge and practical knowledge of the administration of intravenous magnesium sulfate for patients with cardiac arrhythmia. Recommandations : Provide educational and training programs to improve nurses‟ awareness regarding administration of intravenous magnesium in critical care units supported with evidence-based practices, protocols, and guidelines.