Background: Noninvasive ventilation (NIV) is now seen as first-line treatment in patients with acute exacerbation of chronic obstructive pulmonary disease and respiratory acidosis (AECOPD). Therefore, the identification of the predictive factors of NIV failure could assist clinicians in detecting the patients at greater risk of negative outcomes. One of these factors is the diaphragmatic dysfunction (DD). Although the measurement of trans-diaphragmatic pressure is the gold current standard for assessing diaphragmatic function, ultrasonography (US) of the diaphragm at the bedside is capable of identifying (DD) in several clinical conditions. Objectives: to evaluate the prevalence of diaphragmatic dysfunction (DD) during AECOPD, and its impact on NIV outcome. Setting: Department of Chest, Respiratory intensive care unit (ICU), Beni-Suef University Hospital. Methods: a prospective observational study which was conducted on 41 adult patients with acute exacerbation of chronic obstructive pulmonary disease who were admitted to respiratory ICU of chest department Beni-Suef University hospital for NIV. The cases underwent diaphragmatic assessment by bedside diaphragmatic ultrasound. Diaphragmatic thickness fraction (DTF) was calculated from the following formula: (Diaphragmatic thickness at end inspiration – Diaphragmatic Thickness at end expiration)/ Diaphragmatic Thickness at end expiration. NIV outcomes (failure and success groups) was analyzed to find a cutoff point of DTF to predict success  of NIV. Results: The prevalence of diaphragmatic dysfunction in all studied patients was 18 patients (43.5%) out of 41 patients most of them were among NIV failure group [12 (70.5%) out of 17 patients]. The cut off point of DTF was below 0.306 to predict failure of NIV . with a P value 0.002. Conclusions: Assessment of DTF by diaphragm ultrasound in B-mode represents an easy to-obtain new index for prediction of success or failure of NIV in AECOPD patients needing NIV.