Applying acuity scores is an important basis for standardized assignment which allows nurses to perform the care of mechanically ventilated patients correctly. Aim: the study aimed to explore the Effect of applying acuity score on the quality of care of mechanically ventilated patients . Setting: the study was conducted at the critical areas of a JCI accredited hospital in Egypt. Subjects: The study subjects included two groups. Firstly, a convenience sample of staff nurses working in critical areas at the time of the study and having at least one year of work experience in ICU. Secondly, Convenience sample of ventilated patients who admitted in the ICU during the time of the study. Design: A quasi-experimental study design was used in the study. Data collection tools: The study tools were 3 tools including; firstly, nurses' demographic characteristics assessment sheet in which the personal interview was used. Secondly, nurses' practice regarding ventilated patient observational checklist which was used to assess the quality of nursing care of mechanically ventilated patients. Thirdly, patients' outcome assessment sheet which was used to measure ventilated patients' outcome. Results: It calcifies that applying patients' acuity score had positive large effect size on total nursing practice regarding care of mechanically ventilated patients during pre, post & three months follow up.

. Conclusion: There is a significant relation between applying the acuity score and the quality of care of mechanically ventilated patients and thus patients' outcomes. Recommendations: The acuity score should be used in the critical care.