Assessment of fluid status and maintaining its' balance is a very important concept for critically ill patients. The aim of this study: To evaluate effectiveness of structured educational program on knowledge and practice among nurses regarding body fluid balance assessment for critically ill patients. Research design: A quasi-experimental research design was utilized in this study. Setting: The current study was conducted in intensive care unit of Benha University Hospital. Subjects: Purposive sample of 60 critical care nurses were included in this study. Tools of data collection: Two tools were used. Tool I- A structured interviewing questionnaire: consisted of 2 parts to assess: 1) Demographic characteristics of nurses staff. 2) Nurses' knowledge about body fluid balance. Tool II -Observational checklist to assess nurses' practice for assessing body fluid balance: consisted of 3 parts to assess: 1) Patients’ fluid status. 2) Measuring of fluid input. 3) Measuring of fluid output. Results: Knowledge and practice levels were found to differ significantly in relation to years of experience. There was a highly statistically significant difference in nurses' practice between before and after educational program regarding to fluid balance assessment for critically ill patients. There were statistically significant relation between levels of knowledge and practice. Conclusion: Nurses' knowledge regarding to fluid balance assessment for critically ill patients showed obvious improvement and good practice after structured educational program. Recommendations: Setup a project that aims to improve nurses’ performance by implementing evidence based practice. Ongoing monitoring of staff nurses' practice by head and charge nurses when caring for critically ill patients who receiving fluid therapy and provision of guidance to correct poor practices.