Background: Mechanical ventilation (MV) is a machine used to help people who are very sick and having trouble breathing. It can support or take over their breathing for them. Aim of the study: evaluate the predictive value of serum phosphorus level on weaning of mechanically ventilated COPD patients with acute exacerbation. Patients and Methods: This study was a prospective investigational study carried out on 70 mechanically ventilated (MV) patients with acute exacerbation of COPD. According to phosphorus level, patients were classified into two groups , Group I :Normal phosphorus level group (Ph 2.5 – 4.5 mg/dL) .Group II :low phosphorus level (Hypophosphatemia) group (Ph ˂2.5 mg/dL). Results: Patients with normal phosphate level have statistically significantly higher age and BMI than those with hypophosphatemia. There is no statistically significant difference between other vital data, hematological data, ABG data, biochemical data and serum electrolytes level of patients with and without hypophosphatemia. Patients with hypophosphatemia have statistically significant higher rate of difficult weaning and longer duration of mechanical ventilation than those with normal phosphate. There is statistically significant lower serum phosphate level in patients with difficult weaning than easy weaning group. Serum level of phosphate has statistically significant negative correlation with The age, BMI, duration of MV, HR,RR, PCO2 and has statistically significant positive correlation with PH , at cut off level of