Background: The accurate monitoring of fluid balance activities is a vital part of patients‟ baseline information, which guides medical and nursing interventions to achieve physiological stability. Inaccurate monitoring of fluid balance especially in critically ill patients can deteriorate patients‟ conditions. Aim of this study was to assess nurses' knowledge and practice regarding fluid and electrolyte imbalance in critical care units. Subjects and method: Design: A descriptive research design was used. Subjects:  A convenient sample was used (72 nurses) all nursing staff working in above mentioned setting.  Setting: Intensive care units in Damietta General Hospital. Tools: Two tools were used to collect the needed data; nurses 'knowledge Questionnaire and nurses' observation checklist. Results show a defect in the nurses' knowledge and practice as for general knowledge of nurses had unsatisfactory knowledge with Mean±SD 26.8±5.6. for general nurses' practice regarding fluid & electrolyte imbalances monitor and management 97.2% of nurses had unsatisfactory level. of practice with Mean±SD 57.3±7.5. Conclusion: The study concluded that there is a defect in the nurses' knowledge and practice. Recommendations: Continued nursing education and updating knowledge in the intensive care units should be organized regularly.