Background: It is unknown whether body-mass index (BMI) is associated with higher mortality rates and poor outcomes among patients on mechanical ventilation. Purpose of this study: to evaluate the relation between BMI and mechanical ventilator outcome among critically ill patients. Research Design: A descriptive correlational research design was used. Setting: this study was conducted at emergency hospitals in Egypt (General Damietta Hospital). Sampling; A purposive sample of 150 critically ill mechanically ventilated patient  Instruments: Three instruments were used. Instrument one was used for data collection related to patients' demographic and medical data, Instrument two: BURNS Wean Assessment Program (BWAP) checklist. Instrument three: Sequential Organ Failure Assessment (SOFA score).  Results: 76.7% of the studied sample received continuous mandatory ventilation (CMV) mode. Two thirds of them (66.7%) had within normal BMI and difficult weaning from the mechanical ventilators. A negative statistical significant relationship was found between BMI categories and ICU length of stay among the studied sample. Conclusion: No statistical significant relationship was found between BMI and selected mechanically ventilated patients' outcomes. In addition, the current study revealed that the highest percentages of death were found among patients who were in the age group of 40-60 years, males, had within normal BMI. Unexpectedly, the greatest mean length of ICU stay was found among the underweight patients. Recommendation: Replication of the study is required on a larger probability sample selected from different geographical areas in Egypt to obtain more generalizable data.