Background: The accurate monitoring of fluid balance activities is a vital part of patients‟ baseline information, which guides nursing interventions to achieve physiological stability. Inaccurate monitoring of fluid balance especially in critically ill patients can deteriorate patients‟ conditions. Aim: Evaluate the effect of applying guidelines on nurses' performance regarding fluid and electrolyte imbalances in intensive care units. Subjects and method: Design: A quasi- experimental research design was utilized. Setting: Intensive care unit in Damietta General Hospital. Subjects: A convenience sample of 72 nurses working in above mentioned setting. Tools:  Two tools were used to collect the needed data; nurses' knowledge Questionnaire and nurses' observation checklist. Results: The majority of nurses had satisfactory knowledge immediately and after guidelines applications comparing to before. Regarding fluid loss, also more than two thirds of nurses had satisfactory knowledge immediately after guidelines applications and follow up comparing to before guidelines application .There were statistically significant relation regarding scores of nurses' practice regarding fluid & electrolyte imbalances monitor and management pre, immediately after and follow up after guidelines applications    in area of fluid and electrolyte monitor score, fluid and electrolyte management score and total practice .Conclusion: improve nurses ' knowledge and practice after  application of  guidelines regarding fluid and electrolyte imbalance . Recommendations:   Continues education for nurses are required to have the knowledge and competence to care for patients, which includes understanding the indications for and importance of fluid balance charts and fluid &electrolyte imbalance management.